It depends. When you are evaluating overall composition, tonality, color balance, etc., there is no substitute for seeing the whole image, i.e., fit on screen. When you are doing detailed editing, such as making a selection or removing spots, you need to see the image at 100 percent, i.e., actual pixels. For some things, such as correcting CA or dealing with a tough selection problem, I may zoom in to 300 or 400 percent. If you are trying to see what the finished print will look like, a 100 percent view may overly magnify some aspects of the image (for example, sharpening will be over-emphasized compared to what you will see in the print). For that purpose, you probably should be viewing at 25 or 50 percent. When you view the image at odd percentages, such as you might get when you fit on screen, PS's treatment of edges may not be an accurate reflection of what will print.
Tip: If you find yourself zooming in and out frequently, you can open multiple windows for the same image and show the effect of your edits at different magnifications simultaneously.
