Having a squared back is really important, yet I seldom hear anyone mentioning it when giving directions on what to provide for the longarmer. I've done quilts with a lot of seams in the back, and if possible I place them horizontally. I usually float my tops, but I just did two quilts for friends and found that their tops were not square. (One had particularly wavy borders.) I couldn't float those tops because I needed more control over the shape of the finished quilt.