The goal is to keep temperatures at a constant, moderate level (below 75C). It's a good idea to use supplemental fans, like a $20 box fan to keep good air circulation and remove heat around the cards. That makes the card fans work more efficiently, so you don't have to turn them up high. Which prolongs the life of the fans. It's also much cheaper and easier to replace a box fan than multiple card fans. If you have more than 2-3 rigs in a room, you will probably need to look for ways to exhaust the air, so that it's not just circulating the same hot air.