Depends on the mob. There are so many that don't run, that anyone who makes this a quick and fast rule, is often quick and dead.

In my experience, the floating ghosts seem to run pretty consistently, along with a few of the skeletons here and there. Sorta hard to predict but I'll start paying attention to patterns, on my monk.. I just punch.

There is one with spiders :)

The Warders (clerics) will run. All the ghosts will run. It seems only the non-cleric skeletons won't run, and I have no clue why this is.

Same as for crypt nameds, they all run when I don't think they should be... but I have no evidence to back this up.