Occasionally, quilters have the problem of the bias sides of their pieces stretching a little during sewing.There are a few things you can do to minimise this:
Press your material, and don't vigorously iron it and/or starch it when you press it. Sometimes, if you think you will have trouble, you can stay-stitch just inside the bias edge. When you are piecing you can put a pin at the start and the end of the two edges you are sewing together so that you can ease in any stretched edge. Hope this helps.