To measure my quilts to make sure they are square I use place my quilt on top of my cutting mat (36 x 24) that is on my cutting table.
I measure the middle both directions first and then all the sides.
I've not had issues with my own quilts doing it that way. But I have had issues with customer quilts that I longarm and have that extra that I have to tuck in somehow, and to adjust the longarm pattern as I often use designs that are from the computer I have with my longarm. It makes it difficult to get them perfect if the quilt is not squared. I try to measure my customer's quilts before I load them to see if I may have issues as I quilt and make adjustments as I go.
So far I've had good luck in 'fixing' them. And they all look great when they are done.
QuiltingGrannie
Quilter's Pantry