I've had customers actually pull a small metal ruler out of their pocket and measure their art just to see if I framed it straight.
Hanna, you mentioned the customers that bring things back, and mention that they think they look crooked. We had someone take home a frame job, and a week later they brought it back, because "It was Crooked". Guess what? It was EXACTLY STRAIGHT. The problem? The LJ frame was picing up the light differently in her home, then in our shop, and therefore it looked darker on one side, then the other. When we picked up the frame, and turned it to get the light to fall on it from a different angle, we could see the optical illution. She took it home, and hung it on a different wall. Perfectly happy, because we took the time to look at it with her. (AND we never made her feel bad about being wrong.)