I shoot 35mm thru 8x10". In 35mm and MF (6x4.5) I use glassless carriers, the negatrans for 35mm and a twin-plate carrier for 6x4.5. I've never noticed a problem with film flatness, and I've done enlargements up to 20x24" from 35mm (and a test enlargement of 120x from 6x4.5 to see the grain). I've had grain problems way before sharpness was an issue.
For 8x10, I use a glass carrier, because that's what came with my Elwood. I use regular window glass with no Newton Ring problems. However, it's a pain in the ass to clean four sides of 8x10" glass.