My experience with this issue has generally been because acid slowly seeps out from under where the vent caps are pressed on, and eventually the acid makes it's way over to the terminals, causing the crusties (the caps are just a plastic-against-plastic seal, and not a very good seal at that).
Since realizing this some 20 years ago, first thing I do with every new battery I buy that has removable vent caps (except for AGM types) I pull the caps off and wrap 1½ turns of teflon pipe thread seal tape around each cap plug (being careful not to block the vent holes), then firmly reseat the caps back onto the battery. This stops the acid seepage and subsequent terminal corrosion (I don't use grease or felt washers either, no need for them if there's no acid leaking).
Teflon is not affected by the acid, so it maintains the seal well (however if you pull the caps off to check the fluid level, you may need to re-wrap them again).