We were taught to use our borders to square the quilt. When you're ready to add borders, measure through the center horizontally and cut two borders to that length. Find the middle of the top and bottom of the quilt and mark/pin/finger press it, then find the middle of the border and mark it. Pin the centers together, then pin along one side, then the other, matching up the corners. Sew the border strip on with the quilt on the bottom and the border on the top. When top and bottom are on, use the same procedure to add side borders. We've never had anything way out of square.
I don't know what to do if you make a quilt with no borders.
Darren