Rotating or straightening images causes pixels to be recreated from their neighbors.
The canvas size is also affected.
Since this is unavoidable, if possible, I save rotating, distorting,resizing until the end of my retouching just before sharpening. Although very minor, this takes advantage of the softening effect on my retouching (especially cloning and layer masking edges)

The top row started as a 10 pixel square image, rotated left 10°, then back to original.
Second image is larger and used a 45° rotation.
The first image grew by 2 pixels on each rotation and the second grew by 11, then 20.

forum discussion: http://forums.dpreview.com/forums/read.asp?forum=1006&message=25819514