Yea, tubes seem to be able to go south in ever more innovative ways, and I don't believe there's any reliable rule of thumb. Echoing the above, tube performance can be awfully subjective, and trial and error seems the best way to go.
Way I see it, always helps to have a spare set lying around. Bottom line, one day you'll need them, might as well have them. Having them in the meantime, however, permits you to gather all types of useful info regarding when you might need them.
If you've got some offending bits coming out of one channel, the trick is to swaps tubes in pairs between the two channels (in the event you have more than one tube per channel). If the offending bits switch channels, that's your culprit. If you're not a fan of the sound coming from both channels, it's statistically less likely (but by no means impossible...) that you have a "bad" tube, and more likely that you just don't like the sound of the tubes you've got. Or, as 4est notes, could be -- if you have multiple, interchangable tubes per channel -- switching their position in the in the chain might help. As far as tubes, there are obviously way too many alternatives, if tube rolling is your thing, and you should be able to find something else that suits your taste.
If the whole shooting match seems bright, or flat or otherwise not so hot, this is when the extra set of tubes comes in handy -- and the only way I know of to really shed some light on what you're hearing. If monkeying with channels doesn't identify what ails you, swap the whole batch out: if it sounds the same, then tubes ain't your problem, put the old ones back and keep looking (knowing all the while that you've got your spares when you need them). If the clouds part and the sun rises on a better tomorrow, then it was your tubes and replacing them was obviously a good idea.
Best of luck.
