I use a large square ruler to cut/even the corners. The top should not have to be cut, as it should be square. If it's not, them perhaps the rest of the top has problems also. It all starts with making accurate templates or cutting strips accurately. Then seam allowances must be even. Yes, it takes time, but eliminates problems all the way through construction. I cut the batt and backing ¼ inch wider so that the top is ¼ inch in. That way, when I pin the binding in place, its raw edge matches with the top and the batt and backing extend ¼ inch. After the binding is stitched in place, it's rolled to the back and the binding is filled with batt and not empty and flat.
SandyQuilter