Even leveling your camera or framing your image with extra space around it may not be sufficient to prevent the perspective tool from creating a situation where one is left with a choice of a greater crop then desirable or a need to fill in black triangular slivers.
Of course, filling in the black triangles seamlessly with the repair/clone tool assumes that the content in the surrounding areas will be amenable to that process. While I have done it successfully as an exercise, I have never felt the need to use that technique with any of my images.
Mark
