I almost always float the top--sew it down at the top along a line stitching down the batting using my channel locks (so it's straight), then pin the edge to that line. One key to keeping it straight is to make sure not to tug too tight on the side clamps--as a matter of fact, I have my clamps attached to a school ruler that is inserted into a sleeve of canvas about 12-14" square--then I pin the canvas edge to the back/bat. That prevents the uneven tug on the sides plus keeps my ruler table from hanging up on the edges, too. As I roll the quilt I line up the sides using either a long tap measure with markers that velcros on the sides, or even just wood yardstick tucked into the quilt/take up roll and laid along the edge of the quilt. Where the quilt & bat lay over the belly bar, (I float the backing too) I use long magnets to keep it lined up that way. I have found that I'm able to keep it square this way. When I get a super king I will attach the quilt to the bar leaders just cause so much fabric to wrestle.