My quilter requires the backings to be square so I need to square up the backing. I do square before binding. I ended up using a large 15 inch square ruler and my 8x24 ruler (Thanks Tartan) and squared it that way but not sure it is entirely square. Guess I'll go with what I have and see what she says.
Edited, it is at least the same length on both edges when folded in half so guess it will be OK. LOL!!