I've never found film flatness a serious problem in rollfilm, but certainly some backs and cameras are better than others in this regard.
I think the quality of the engineering may matter more than whether the back winds with the curl or against it: my least flat (but still perfectly usable) backs are old Graflex ones, and among the flattest are Horseman / Arca ones - both wind in the 'against the curl' way, like a 'blad.
My very flattest-film cameras are 'with the curl' varieties: Rollieflex and, best of all, Fujica rangefinders. The Fujicas are similar to the Pentax 6x7 in the way they wind the film, and the flattness the big Fujicas manage across the whole of a 6x9 frame is extremely impressive.
Peter