The F3, F4, F5, and F6 are the only Nikon film SLR models that show 100% of the frame in their viewfinder. The F100 shows 96%. The N90s/F90x and many other models show about 92%. A few bodies show as little as 89 or 90%. What's this really mean in terms of composition?
If you're shooting slides, 100% viewfinders are actually a bit of a nuisance, as most slide mounts "crop" a bit of image, hiding the edges of your photo "under the mount." Unfortunately, slide mounts vary a bit in the amount they cut off, in my experience ranging from the equivalent of a 92% viewfinder to about 94%.
At 92%, you lose almost 3 millimeters across the width of the image. This is significant for two types of photographers: (1) those who make large prints (losing 8% of the image area can be the difference between being able to make a clean 16x20 versus a 11x16); and (2) those who like to place objects at or near the border of the frame (having an object misplaced by 3mm is not trivial!).
My advice if you shoot slides: don't worry too much about it. If your viewfinder shows less than 94%, crop a little more tightly than what you see and be prepared to occasionally have to remount the image in a 100% mount. If your viewfinder shows more than 94%, frame to what you see and know that you may be opening the mount to get the full image, when necessary.
If you shoot negative film but rarely print larger than 8x10, just frame what you see in the viewfinder. But note that the printer probably needs cropping instructions from you to match what you framed.
