A question because I rarely use extreme wide angle (for the format) lenses. On 4x5 and using 65mm to 90mm lenses -- would there be any distortions caused by the lenses that might have to be taken into consideration if using the top (or bottom) half of the projected image from the lens vs. using the center half?
That is a good question, that I probably don't have a great answer to. My hunch is that for less well corrected lenses wides you could have some pin cusion distortion, or whatever the reverse of that is called (where the bending deflects toward the center not away from the center.) Using only the top or bottom half would probably mean that the distortions would all be in the same direction rather than the top bending up and the bottom bending down--everything would bend up or everything would bend down. But if you are using a lens that covers 4x5 not centering the lens over the upper half or lower half is just showing you that half of what a straight 4x5 shot would show, so you're not moving into the far reaches of the image circle.
Probably the biggest issues (to my mind) is that any distortion in the lens may no longer be centered--but againyou'll see the same effect if you use movements. the most noticable, especially with small image circle wide is probably that any vignetting won't be consistent. I'd worry about that more with a 65mm or 75mm lens than I would with a 90 or 100, but if you are shooting chromes it could call for a center filter (and they are getting hard to find and expensive.)