I can't be arsed to roll tons of monster hit dice, when I could be rolling random encounters instead!
I'm with the crowd that says NPCs and monsters heal whatever I feel like they heal. That said, they usually have as many HPs as I feel like they should have (I might let an ogre drop after a hit even if it technically has 5 hp left, or visa versa, let it stand even if it has -5 hp...the story is the thing)
Why even have the PCs roll, just tell them what result you want?