I am all but ready to submit my images for review (my nerves are SHOT at this point LOL), but I have a couple of last minute questions.

How big of a no-no is it to mix keys, for example, if the floor is dark brown, but the wall is light?

Also, people keep saying to beware of a centered image. Some images don't make sense cropping them off to one side or the other, nor would the client want it that way. Should I really avoid any center cropped images all together?