I have a smaller - 24x36 rectangular frame and I baste prior to putting a quilt in there because i noticed that the quilts seemed to shift as i quilted and my corners always came out wonky. I always start in the middle and work my way out. When I baste the quilt (I use thread, like it better than pins) my quilt doesn't shift and my corners remain square.
The quilting groups i work w/ all have the frames that you roll the quilt on - we don't baste those.