I photoread in bookstores standing up all the time. It is especially good to read in those Gigantic bookstores. That way you don't stick out and remain anonymous (plus the selection of books is hard to beat).
One thing I notice standing up is that my knees and joints hurt after a while. I don't notice it while I am photoreading but I do when I go to leave. Its like I am so intently focused and in state that I don't move at all. Being in a locked position like that for a long time can create a little stiffness.
