In my experience, the PCs fleeing combat is one of the rarest events in a D&D campaign. It often seems like they would rather fight to the death and suffer a TPK than turn tail and escape. This happens even when escape is relatively certain, and even when the fight is largely unnecessary.
It happened most recently in my new campaign. (Note, the PCs are starting at 3rd level, not 1st.) The first room in the dungeon contained 2 chuuls -- a deadly encounter for the 5 PCs -- and an easy way out that the chuuls could not follow. Despite bad luck -- not hitting and taking hits themselves -- the PCs absolutely would not take the way out. They focused fire and dropped one of the chulls, so I gave them a free round while the other devoured the innards of its mate. Even then they did not take the opportunity to leave. Injured and already almost out of their big guns, they decided to face the thing down, even though they have not hit it yet (so it is fresh).
Could be the shortest campaign ever, I guess.
Anyway -- what is your view on PCs fleeing fights? Do you see it happen relatively regularly? Do you design encounters to make it necessary? What is the GM's role, if any, in the party deciding to stand or flee?