Originally Posted by elTwitcho

Not only can too tight ruin the crank arms, but too loose will ruin the crank arms. So can re-tightening the crank arms after they're already installed.

This is why a torque wrench is a good idea. A 20 dollar tool can save you from ruining a couple hundred dollars in cranks. You can get by without one on cheap cranks (I installed a 50 dollar set of cranks without a torque wrench because I didn't mind if I messed em up) if you just tighten them down really well. It's a lot more common to have them too loose than too tight.