It depends on who they were fighting... a bunch of random chumps are usualy left to rot on the ground. Named NPCs sometimes have their heads taken (if there is a bounty). Enemy spell casters are always dismembered and have their parts scattered about (never trust a wizard to stay dead in his grave).
In a recent game they killed an ogre blood sorceror and took his head back to a villiage he had been tormenting, it was still bleeding when they left several days ago.
In a recent game they killed an ogre blood sorceror and took his head back to a villiage he had been tormenting, it was still bleeding when they left several days ago.