I like 6x6 for square landscapes, but another way to think about it is that with 6x6, you can crop the frame any way you want, so it’s like having a view camera with rear rise, fall, and shift, but without the apparatus. If you want the camera level, so all your trees are nice and parallel, but you don’t want the horizon in the boring middle of the frame all the time, then level the camera to keep the verticals straight, and crop for a horizontal image to put the horizon where you want it—top third, bottom third, wherever it looks right.
If you use a 35mm insert or 645 back on a 6x6 SLR, to me, that’s like losing the option of moving the horizon while keeping the camera level.