My blades were curved such that they were shallow angle on the leading edge then curved to be more of an aggressive angle (ie facing the direction of travel) on the trailing edge. This meant that if I ran it one way it would be efficient, scooping the air and shoving it out the back, but if I reversed it the leading edge would be of a steep angle then the trailing edge would be at a shallow angle thus just making turbulance more than proper air movement.
If your blades are curved as above, then only run it as above (fan would be turning anti-clockwise if you were the single arrow in bottom left). If you ran it t'other way it wouldn't be as good.