I don't really promote my units at all. I think that early promotions like shock and drill kinda suck and for the really good promotions you need shock/drill 3 first. Also getting lots of xp for a single unit is too hard without honor tree double experience or abusing a xp farm which I find too tedious. Another thing to remember is that ranged units promotions become useless once you start upgrading them to riflemen. Bottom line is that I almost always save my promotions for the insta-heal. But maybe other people have better experiences of highly promoted units. So are there good promotion paths for your various units (melee, ranged, naval)?