Its something i've been wondering too.
In my experience 5 stacks of Valor Buff gives + 75% MF en GF on bosses and mobs.
So i think it works equal on bosses and regular mobs.
'Normal' MF works on mobs and bosses.
I think many ppl are confused because a lot of farming strategies involve getting 5 stacks from elites and then go to a boss to get the best loot.
In my experience farming elites in act 4 Hell or Act 1 Inferno with 5 stacks and clear whole maps is even (or more) profitable but thats your own choice ofc :).
I hope this answer your question, but some other ppl might add something or correct me.