Bill, I'm surprised that a purist like you isn't into shimming rotors. Shimming and lubrication of the hub face are of course not required, but there's nothing to say you can't if it's done half-properly. You of course want nice clean faces on both the hub and rotor. I'll use a wire wheel cup on a drill if needed. For new and uncoated rotors, wiping with a very light coat of oil or high temp brake grease to help avoid future rust isn't at all a bad idea as long as you avoid lubing the braking surfaces and threads (wheel nuts are supposed to be torqued dry). In most cases it will quickly disappear with road spray and dirt, so probably not worth the effort. Regarding shimming, here's from some old brake notes of mine:
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What is important is staying within the overall lateral runout spec. With quality rotors and lower mileage hubs, most shops won't even bother checking the runout unless someone is reporting slight brake pulsation on new rotors. With cheaper rotors and/or worn hubs, a decent mechanic should be expected to at least index them, meaning to mount them on the studs in all possible orientations to achieve the least runout. With the 10 mm Volvo wheel pins used as the basic rotor mounting bolts, that may mean only two possible orientations 180 degrees apart for some brands of rotors. If desired and it really made a difference, you could run without the mounting bolt, or you could do like I do and resort to shimming, but only if and when needed.

There's always going to be some rotor lateral runout and purists like me will want to minimize it to help with more even braking and brake wear, and especially to help avoid brake pulsation both initially and down the road. With new brakes you can tell it's there when you turn the rotor by hand and hear/feel the pads scuffing unevenly around the rotor. A small amount won't cause problems and may actually lessen over time, but larger amounts of runout can make for uneven braking and uneven hot metal expansion that can lead to noticeable pulsation and ultimately make the runout worse, which we call warped rotors. What many call warped rotors usually aren't actually bent. It's mostly that they're unevenly worn for thickness, which makes them measure as warped. If you measure thickness around the rotor looking for the thickest and thinnest spots (as I do), these spots normally correspond side-to-side with the high and low runout spots to say they're not actually bent.

There are two sources of lateral runout: runout of the hub and runout of the rotor. Hub runout is a combination of slightly bent hubs (which can happen with an older car with rough roads and potholes) plus the runout in the original hub machining. Rotor runout is two-fold as the runout of the machined hub mating surface and runout in the machined braking surface. Combined, this all makes for the lateral runout measured near the outer edge of the rotor face in reference to the axle hub mounting with a dial micrometer. Runout is supposed to be measured a nominal 0.15 mm or 1/16" from the outer edge, but 1/8" is generally acceptable for rotors, and for used rotors you may need to measure in a bit further to get past the ridge. The dial micrometer base needs to be fixed in relation to the axle hub mounting, not the body, suspension or ground. I use the front strut and the rear axle housing.

The Volvo spec for hub runout (both fronts and rears) is 0.03mm (0.0012"). For worn wheel bearings, you need to measure this keeping the hub uniformly pushed in. Technically, a hub that exceeds this should be replaced along with the bearings, but if it's just a little bit over then rotor indexing may be all that's needed, in fact that's what the Volvo manual suggests.

The Volvo spec for lateral runout depends on the model and brakes used. Specs can be found in Chilton's Volvo 1990-1993 Repair Manual (p 9-31) and others, but I've found they're not as tight as the numbers I've found elsewhere, such as in the Volvo green manuals, the various Pocket Data Books (as used below) and the Volvo TSBs that may have tighter tolerances.

o When starting with old rotors, the first thing to measure is thickness (generally requiring caliper removal). Measure just inside and safely away from any outer ridge and avoid score lines. If less than the minimum thickness then it needs to be replaced (it varies by type of rotor, sometimes stamped inside or on the rotor, sometimes not, too many to list here, see the charts).

o Next with old rotors is to note the thickness variation between minimum and maximum thickness around the rotor. This is actually the easiest and best indication of whether your rotors are "warped" to confirm they're causing brake pulsation, but does require careful measurement and interpolation between micrometer marks. I use a felt pen to mark the rotor.
... For the fronts, it's 0.008 mm (0.0003") for all later 700/900 single piston Girling brakes. For 240s, it's 0.01 mm (0.0004") for the '79-on ventilated rotors and 0.02 mm (0.0008") for earlier ventilated rotors, with 0.015 mm (0.0006") for the very early solid front rotors.
... For the rear rotors, the max thickness variation for the later 240/700/900 RWD is 0.012 mm (0.0005") for solid axle and 0.008 mm (.00032") for multi-link rear axle suspension (760/960).
... Basic rule of thumb is the thickness variation should be less than one half a micrometer division (0.0005") otherwise you need to measure more carefully or just go for new rotors.

Now for checking and indexing the rotors, here's the maximum lateral runouts you're looking for (and hoping to do better):
... For the fronts, it's 0.035 mm (0.0014") for both ventilated Jumbo Type I (287 mm dia) and Jumbo Type II (280 mm dia) used in the 700/900s. For 240s, it's 0.04 mm (0.0016") for the '79-on ventilated rotors and 0.05 mm (0.002") for the earlier ventilated, with 0.04 mm (0.0016") for the early solid front rotors.
... For the rear rotors, it's 0.05 mm (0.002") for solid live axle in the 240/740/940s and 0.04 mm (0.0016") for multi-link rear suspension (760/960)

If indexing isn't enough then shimming can be used to get them running even truer. I've made cardboard crescent templates with holes for the studs (one two hole, on three hole) that I use to cut shims from aluminum foil. They must be kept flat, not wrinkled. Sometimes 2-3 layers are needed, and for a really bad rotor I've gone as high as 5 layers. To make sure they're fully compressed, I'll often over-torque the wheel nuts a bit, then back off and re-torque to spec. When measuring runout, I've made 1" spacer collars out of metal tubing to place over the studs and be able to tighten the nuts close to spec.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now