Carla,
I square my quilts after quilting. I take two measurements. One in the center in both directions. Write these numbers down. Then measure each outside edge along the borders. Compare these numbers to the ones first taken. They should be very similar. At this point you can do minimal trimming.
Using the shortest measurement from the two opposing sides, measure your binding piece to match this measurement. Your binding piece should be yards long so do not cut it, just make your quilt fit within the shortest measurement. Fold the binding in half and match it to the center point of the quilt and pin; then into quarters, etc, easing to distribute any fullness. Stitch, make the corner mitre and then start the process over again for the next side.
Do not start your binding at a corner. Attempt to start it in the middle of one side using only half of the measurement and the balance when you get to the last side.
You will be able to test how well you did once the binding process is complete by bringing the opposing edges together at the corners and see if they are the same. (Incidentally, judges for quilt shows do then when judging.)
Some quilters just slap their binding on and never bother to measure. This is why their projects are wavy when hung.
Lastly, I only use bias binding on an edge that is curved, otherwise I use a double straight-grain binding. The formula is 6 x the finished binding width + 1/4". Example for a 1/2" binding: 6 x 1/2 = 3" + 1/4" or strips cut at 3-1/4". (The 1/4" is taken up in the thickness of the quilt. If you are using a thicker batt, increase the 1/4".) Once all of your strips are connected in one long strip, press in half lengthwise and then align the raw binding edge of your quilt project. Stitch a scant 1/2" on the right side of the quilt. Press the binding away from the body of the quilt and when you get to stitching on the back side, your folded edge is ready.
I know this reply is lengthy but hope it gives you another option. Suzanne