Does a Pulley have to Be round?

I was just wondering how much the shape of a pulley affects efficiency.

Im running flat belts and am considering using some hex stock as one of my pulleys but im not sure if it will be a problem

The best reason I can think of off the top of my head why pulleys are round (as opposed to other shapes) directly after "they're easiest to make that way" is to not have fluctuations in belt tension. With other polygonal shapes you'll end up with a very inconsistent transmission system that in the worst case will in fact eat the belt or put excessive strain on the motor (switch from running with no load to running at full tension in fractions of a second).