IMO, there's only 1 criterium : the end result... If you achieved that "perfect" shot by "helping" nature a bit (e.g. rearranging a fallen leaf or moving that shell 10 inches to the right) then I won't think you're trying to cheat me. Now, I never felt the need to "arrange" a photo, but I wouldn't feel guilty about it. I find the pure "objective" approach really very restricting. Now, if we were discussing about extensive use of postprocessing techniques...