That really depends on the assumptions of the game and setting, doesn't it?
In most of our games, we are adventurers, and picking and choosing our fights is the job description. If we strode boldly into every potential battle we've run into, we would all have died ages ago.
Oh, my campaigns are the same; the PCs often meet things too big for them to take in a straight-up fight. But when I put those encounters in, they aren't designed as combat encounters. The OP was talking about "throwing [an encounter] at the party," which to me reads like, "the monster attacks them."
What I meant was, when battle starts, PCs stride boldly into it... I've known hyper-aggressive players and cautious players in my time as a DM, but once battle was joined, they were almost all reluctant to back down. If they didn't want to fight, they didn't start the fight in the first place; if they underestimated the opposition or the DM forced them into battle, it usually took one or two deaths before the rest of the group decided to run.
The rules of the game encourage this sort of behavior. Fleeing characters are usually subject to opportunity attacks, blasts from the enemy artillery, and other such penalties; plus it means leaving behind unconscious characters to die who could otherwise have been stabilized. And the hit point model makes it very hard to determine which way a fight is trending until people start dropping.