I wouldn't square up by cutting because your quilting in the border itself will change the square a bit. I would probably mark the binding line as squared up based on your last border seam and stay about 1/8-1/4 inch inside it like Tartan said.
I am actually almost at this spot on a queen sized quilt and trying to decide my quilting pattern. I am thinking of marking inside by 1-2 inches, quilting my feathers/swirls, then doing a row of pebbles and then something outside that to cut off..... Or I may just feather and try to stay close to the binding edge.