I fell into this trap again of making my icon for a iPhone app l am currently working on not exactly 57x57 pixels. I have the height set at 57 but the width because of the auto resizing in Photoshop it was suggesting l make it 54px, thus when l looked at the logo it never looked right as the bottom of it was cropped off.

Remember people iPhone app icons must be 57x57 exactly and if your image looks wrong at those sizes well bad luck fix it