go ahead and sandwich it and quilt it- then square up the whole thing- since the quilting process draws the fabric up some and distorts the quilt top too it will save you a step if you just go ahead & quilt it first- then lay it out- I sometimes will use a ruler & pencil or chalk- line up your ruler starting at one corner- draw a straight line- in both directions- line up & draw straight lines all the way around the quilt- getting everything nice & straight- then you can use either scissors or a rotory cutter and cut along your straight lines to square up the whole quilt before binding.