All sheet film sags a little unless you have something like a vacuum back. Even lying flat, if you look at a reflection of something linear like a
fluorescent light bank, you can often spot a bit of waviness. More a problem with thin films and large film sizes - but the shorter focal distance of comparable perspective lenses with these smaller film sizes just makes it all fussier. I've also measured true focal plane positioning with even expensive view cameras using a specially built depth micrometer, and about all it takes is a tiny bubble in the varnish, or a little burr on diecast alum to make an issue. Ideally, one would not want to stop down a lens for this size film as far as with a 4x5. ... 6x9 is different enough to make a real difference, and 645 way down the food chain. Different strokes for different folks. But if you're going to the trouble of taking along filmholders, might as well make em 4x5 to begin with. 2x3 sheet film will be just as much fuss, and only a modest wt
savings. A properly made roll film holder will have no curl in the image area. A lot depends on one's expectations. My brother did a lot of
Tecknika 6x9 work back in the 60's, and only about half of it ever came out really sharp. I don't know how much of that was due to focus
issues, and how much due to less than ideal holders. I recently made a number of 16x24 enlargements of 6x9 Ektar roll film, which is pretty
insane by my own personal standards (I rarely enlarge more than 4X - and NO I don't need any comments from someone who tells me how they create inkjet smudges four feet from MF film!) and at that degree, any little glitch becomes a real problem visually. It's just hard to make small film resemble 4x5 work, let alone 8x10, but it's something I'm seriously experimenting with so that I can keep backpacking with
a view camera in old age. The days of lugging a 90 lb pack with a Sinar system in up the peaks and passes has ended. I'm not a fifty year old
teenager any more!