I have around 10 sets of zippers. I just pin to the zippers (I used to sew the back to the zippers with a big stitch, and will still do that if I'm prepping a bunch of quilts when someone else is on the machine). I don't float the top, I like the control that rolling it on the roller gives me, and I have some warning that things are not square and straight if the quilt top doesn't roll straight. When loading, I measure the top somewhere across the middle of the quilt. I use those measurements to attach both ends and the middle of the quilt on the zippers/leaders. If there is extra fabric in the borders, this is where it really shows.
On tops that are critical to keep absolutely square, I use a lazer square that I bought at the hardware store. If the rows of blocks are, say, 10", I measure down 10" on both sides, put pins straight up and use the laser to make sure the row is absolutely straight.