Does rotating a photo count as a destructive editing?

Question

I usually rotate all my jpg photos (using Windows Photo Viewer) after removing them from my camera's card. Is this correct? Am I losing quality for rotating them? Should I just leave them as they are and later incorporate the rotation as another retouch step - at a post-processing software like, for instance, Lightroom?

Sometimes I see websites like DPreview.com showing their camera's sample photos not rotated, thus I thought rotating them would reduce (a lit bit) their quality. It it a fair assumption?

Answer

Whether rotation loses quality or not depends on software (and its version) used and image dimensions. Images that have width or height (measured in pixels) not divisible by 8, cannot be rotated in a lossless way.

As for Windows Photo Viewer, Matt Grum has already given a stellar answer to the more specific question. For other software, you could use the sample images in his answer and try the same experiment.

Check more discussion of this question.