It seems that every couple weeks I see a post somewhere about someone's tachometer jumping around on the range, possibly reading 0 at times, despite the engine seemingly operating normally.

This is actually one of several syptoms that the distributor is starting to fail.

I myself have had this happen, and I ignored it for a while. After a couple more weeks, the engine started misfiring badly when I went around corners.

A lot of the time, if a distributor is going bad you can grab the shaft and shake it. If there is play, that indicates the distributor is failing. However, from what I've seen of the cases where the tachometer jumps around, there won't be any free play. For those not wanting to spend the money on a new dristributor because they are a bit on the expensive side, I do strongly suggest picking up a used distributor from your local junkyard, or ordering a used one from the junk yards listed in the resources section.