I always float my top. That's actually how I was taught. I like it because I can lift it up & check seams or easily smooth out the batting. I have some storage tubs under my frame, so if the top is so long that it will touch the floor, I just accordion fold it & lay it on the tubs. Occasionally, if the piecing is uneven and I need to add some tension, I will just use some bar magnets. If you have a harbor freight near you, these are the ones I have. I made a muslin sleeve for them. This will only work if your rails are steel, however. Aluminum won't work.
http://www.harborfreight.com/18-in-m...der-61199.html