Do you square your quilt top before adding borders?
Once your piece is quilted, you need to make the edges even and square before adding your binding. It is easier and more accurate to trim the quilt sandwich if the three layers are sewn together at the edge. Otherwise, the bottom layer can shift and you end up with edges that are not the same.
How do you know if a quilt is square?
To answer your question specifically…
Another way to see if your quilt is square is to fold the side edges into the center and compare. If the edges are within a 1/4″ of each other and the center, you’re pretty much good to go. Repeat with the top and bottom edges folded into the center.