...or this might be a good subject for a poll.
My philosophy is to test for anything you might be adding to the aquarium otherwise you won't know if you're adding too much or too little (the engineer in me). Therefore, if you're adding kalk/calcium reactor/B-ionic you'd better be testing for alk and calcium (if you're using kalk you should test pH too). If you're adding iodine, magnesium, other trace elements you oughta be checking for them too. Perhaps in an established aquarium when the additive rates and consumption rates have stablized you could get away with not testing (I'd say still test, maybe not as frequent). The other thing I test for occasionally is pH and nitrate.
The other tests (silicates, phosphates, O2, ect) I've never used.