Yes, the camera will be in manual mode but it isn't about the camera changing anything it is that after 500 shots, is the camera calibrated well enough to
hit f5.6 (or w/e) without fail? The aperture on my G-lenses is at their maximum until the camera actually takes a picture. That said, in theory, if things in the camera are extremely precise, then I might get one picture that is f5.6, one at 5.7 and one at 5.5. Over the course of time, this would be very noticeable that the aperture is changing ever so slightly over 500 pictures.