It was OK to run away from monsters, we did so a lot. I remember losing my favorite character to a random god from Deities and Demigods.
One thing I've told my players is that I don't set up encounters designed for them to win all of the time. They need to develop the skill of watching their resources and evaluating what is the proper action as the encounter progresses.
One of my dungeons nearly got derailed by a troll. We were playing OD&D, where they got multiple attacks against 1 HD creatures but only 1 attack against anything larger than 1 HD. They had been wading through monsters and were stunned when they found out how tough the troll was. (They didn't know it was a troll, and didn't realize it was regenerating.) A near TPK but a few escaped and exited the dungeon. They went to town, healed and recruited more characters, then went back in. Similar result. It took them four trips to the dungeon to get past that one encounter.
This never happens in dungeons where players are supposed to win, which is a shame. Hitting a tough obsticle really got them thinking and not just randomly rolling dice. Folks ask me if OD&D is a combat game or a role playing game, and clearly this shows how it can be both. DCC is a lot like this, philosophically. When you play you have to know you could win quickly or get squashed and have to be able to look for clues as to which will happen when.
I might also point out that my players never quite know how strong a monster might be. I might pull it from my OD&D Monsters & Treasure book, AD&D Monster Manual, C&C Monsters & Treasures, the Rules Cyclopedia, or wherever I happen to have handy at the moment. The number of hit points, armor class, and many details vary from book to book and so do my monsters.
At least, that's how I play.