It really is personal preference. If you've been wearing glasses daily for a number of years it is probably easier to keep the on when photoreading. If you only use glasses for reading then it is quite possible that during the photoreading stage you don't need to wear them.
Each person has a different reason for wearing glasses in the first place so there is no one right answer.
Try it and see, consider; your personal comfort, ease (if page turning becomes difficult because you are not reaching the right point of the page you are losing the ease of keeping the rhythm), results you obtain after photoreading with your glasses on or off (i.e. do you feel the book coming together faster after photoreading with the glasses on or off).
Some people will naturally just learn to photoread with their glasses on for the sake of convienience (not having to find somewhere to put them when photoreading in the book store or library. Or finding them again after photoreading )
Alex