I only look at the picture right side up. For my own purposes either it works or it doesn't, and its orientation doesn't matter much to me. Tell me, those who do look at it upside down, what does it do for the final print, once you turn the print back around again?
When I evaluate a print I always look at it in exactly the same location, under exactly the same light, in the same tray of water, so that there are no undue lighting effects compromising how I view the tonality of the print. Since I always print on the same paper, I know what dry-down does compared to the wet print, so it's a good way for me to consistently judge the prints.
In all, I look at how the print values support the content, the gesture, the frame, and the mood. It's such a fascinating process.