Float the top. I just finished a 95 x 95 quilt, and floated the top. I even float the batting. Each time you roll up after your rows of quilting, lift the top, smooth out the batting and then lay top back down and smooth out, baste down the sides before quilting in the center. This will help keep it smooth & square.
Also, after laying your batting ontop of your backing, smooth it, then lay your top on that. Then baste across the top of your quilt top. Starting in the middle, baste to the left, then repeat and baste to the right. This should help keep it more square. Remember that after if comes off the frame it will probably look square.
Good luck!