I floated a top the first time I longarmed. But I no longer do. I quilt so intensely that it would draw the quilt top up in the center of the bottom edge.
I also have an easier time getting blocks to line up on unruly quilt tops that have extra somewhere in the quilt top. I do a lot of charity quilt tops that others have made.
We had a professional judge at our local quilt show last fall. The comment that showed on my slip time and time again was how square my quilts were and how straight the lines were. I know the width of the top of the quilt is the same as the bottom edge because I have measured it and pinned the quilt to the leader so they are the same measurement.
Also if you SID on a horizontal line you can tighten that quilt better to stretch the top a little so you can see that ditch.
I have floated one quilt top in the three years I have had my machine. I was being lazy that day. It was a small charity quilt