Ron,

We're both right.

The PCV valve is always open a little bit except when closed by positive pressure in the intake (backfire, as you said). But it's not an on-off affair -- at high vacuum (idle / off throttle), it's very restricted, allowing just a slight bit of crankcase air through (blow-by is minimal under these conditions, so there's not much need to vent the crankcase aggressively). This should have almost no affect on idle mixture or speed.

Under low vacuum (full throttle / maximum blow-by) it opens wide, but there's still very little flow through it simply because the vacuum is low, so again there's negligible impact on tuning.

So, I don't know that this is a common failure mode, but if the valve were to somehow stick in the wide-open position under high vacuum conditions, you'd have significant false air through the crankcase.