It is always a good idea to square your quilt top before applying the borders. Taking time to square the quilt top will save you much heartache later on. Here are a few suggestions on how to square your quilt top. First, press the quilt top. Press on the wrong side first so that you can get all the seam allowances lying flat. Then turn it over and press the top side.
Now that your quilt top is completely flat you can find out if it needs squaring. Measure each side of the quilt top and also measure the length down the middle of the quilt. If you’re lucky these three measurements will be within about 1/2″ of each other. If they are, you can consider your quilt square and use the average of these three measurements for the length of your border strips. Repeat the process with the measurements in the opposite direction across the quilt top. Measure across the top and the bottom and across the middle of the quilt top.
If these measurements are off by more than 1/2", your quilt top is not square! You will then need to decide what is the best way to square the quilt. You might need to trim a little off of the top or bottom. You might find that some of your blocks aren't the right size. You might find that the fabric has stretched due to the handling. You might need to resew some of your seams on your quilt blocks.
When you decide you have a square quilt, then you can add your borders. This will also help the person that is doing the long arm quilting service also.