A compression test should be done whenever you think you see:
-Excess crankcase pressure, this shows up as dipstick popping, oil smoke in the exhaust, some other stuff
-Low power
-Signs of headgakset failure, or alot of other things
I dunno, if you had a compression tester it would be good to do it every second month or something, more often during race season. If you find a problem, or know there's a problem and a compression test didn't find it you should do a leakdown test and find out exactly what the problem is. Compression tests more or less only show ring failure, or a seriously burnt valve. Stuff like valve seat failure won't show up in an obvious way in a compression test.
On the comp test note does anybody have a brand/part number for a decent compression tester that fits our heads? I've got a craftsman one somewhere that doesn't fit... Something like craftsman quality would be great. Doesn't have to be Snap-On but I don't buy garbage tools either.