You can also sew a row of stay stitching just under a quarter inch from where you want to cut. Reduce your stitch length a bit and make sure you don't stretch while stitching. Then you can just cut the excess off even if it's on the bias because the stay stitching will keep it from stretching. When you add your borders this will stabilize everything. For added stability, you can also heavily spray starch the area you are going to cut first.