'The right ammo for the right target', a good mantra: Troops - MGs or (if there are no enemy AFV threats) HE ITT; * vehicles - MGs or HE VTT (overkill, save for those AT vehicles); BU AFVs that are 'killable' - VTT with AP (special ammo only if you want the extra shot chance, and there's nothing 'bigger' you want to save that ammo for); CE AFVs that are unlikely kill, but still killable - HE VTT (with fun collateral attacks v crew) & MGs (for pesky crews); BU AFVs with ridiculously-solid armor, OR OT AFVs with strong armor (not too many of these, but some of the later German TDs are, as well as most US TDs), HE w/ATT!
Nothing is 'written in stone'...there are, and always will be, exceptions and conditional deviation from the above...that's the 'art' of the game, but it's useful to run the numbers from time to time.
Note, ATT is a good option in the Advancing Fire Phase, as it's unlikely (unless a stabilized gun) that you'll be getting a hit, you have zero ROF anyways, and you'll still get acquisition you can 'bracket down' in the DFF (or in his movement phase when he expends a start MP). If he's in Motion, however, and you want to track, you'll just have to roll the bones with a VTT attack, and hope for the best. ;-)
