I float my tops, not because I think it's better, but because I'm lazy, i guess. I use the quilt clips (I think that's what they are called) to hold it down and provide some tension on the top, if needed.
can you use the quilt block size as a guide for your pattern height? then you know that things will line up equally from top to bottom as you approach the end of the quilt. or, if you intentionally start your designs off the quilt edge, then you can continue to do so across the whole quilt and it will all look even that way.