Wow, thanks for the quick responses!
By "not full-frame" do you mean the entire area of the polaroid is not exposed? So in addition to the already-occuring white polaroid frame, a part of the negative will not be exposed?
In terms of options... I seem to have many options
Most of the polaroid backs for medium format cameras take standard peel-apart film such as 667, 669, 655, and so forth, which is if I remember correctly 3.25 x 4.25 in recordible area. Course when you consider 6x6 for example, only 2.25 x 2.25 of the polaroid sheet is going to get exposed, and smaller for 645 cameras.
One nice thing about the Mamiya RB67 and RZ67 polaroid backs, is due to the RB's "Rotating back", there is 7cm of clearance not only horizontally but also vertically, which allows for 6x7 negative to be shot in portrait or landscape just by rotating the back. But this also means that by putting a polaroid back on, you get a 7x7 cm square image on the polaroid leaving just a small strip on the edge for the extra inch or so thats not covered, but need less to say a 7x7 square on a sheet of polaroid film is about the best you are going to get, shy of finding a polaroid back for a 6x9 camera (or using a pack back on a 4x5 view camera.). I also use a pack back for my crown graphic which takes the same type of film, except in the case of that, the entire polaroid film gets exposed (even though cropped since you are working with a 3.25x4.25 film area inside of a 4x5 coverage).
So while it's nice the RZ/RB will cover most of the polaroid sheet, regardless if you use a 645, you are still going to be exposing a sheet at a time as opposed to getting more shots in a pack, so it can go either way in terms of looking at it.