definitely, change the timing belt religiously; i do mine every 60k; and it wouldn't be a bad ideal to change the seals and tensioner every other change; and maybe even the crank pulley...my crank pulley broke at 167k; my '84 240 has 220k for now...i had my valves adjusted at 150k just because it was recommended; they were dead silent at the time and i didn't notice any difference in sound or anything afterwards; i guess there is the chance of them sinking; but i won't mess with them again unless they're noisy...good luck

It depends on if you are massively inconvenienced by broken timing belts or not. Volvo recommends 50,000 miles on the timing belt. I go 45-55k depending on what the best time to do it is. It's such an inexpensive service that it is silly not to do it. Unless, that is, you have excellent towing insurance :)
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Chris Herbst, near Chicago, IL. 93 940, 91 240, 90 240, 88 740, 87 240

As far as the TB goes, the only reason not to wait until it breaks is so you won't get stranded. It won't hurt the engine. The valves are a different story. Any work on the head can get very expensive. I had the valves adjusted on my '88 740 for less than $100. I think it was around $80. Well worth it in my opinion.

You should most certainly replace the timing belt before they break. Every 50,000 miles is the recommended interval.

The valves: have them checked. My guru told me the valves seldom need adjustment.

I think most would worry about an exhaust valve thats to tight. Would not be good for the head. Or the valves. Of course, out of spec valves, exhaust, or intake, wouldnt desireable.