Explosive Ive found to work relatively well (certainly better then round against troops) The problems with Howitzers is they arnt very accurate at extreme range, and there even more inaccurate at close range.
Mortars I havnt really used, same with percussive and explosive ammunition, so I cant really comment on them, but the key to getting use out of your howitzers is positioning, so that you can keep them firing as long as possible at the best range. You cant throw them to far back behind your line, otherwise enemy troops coming towards you will only be in best firing range for a brief amount of time, at the same time you cant stick them to close to your front line for the same reason.
Just think about the kind of battle your expecting, and where you expect the enemy to stick around the longest.

But yes, in a field battle, with direct fire I switch on round shot at the beginning, making sure i know the limits of shrapnel, and switch over to shrapnel asap. For my howitzers, typically i leave them on explosive (but as I say, I havnt unlocked the other types yet)