First, don't press the way you do with garments--opening the seams. Instead, press both allowances to the same side, typically toward the dark fabric. When you are piecing blocks together, it is nice to have the seams pressed so that they snuggle and don't have too large a lump. So, one seam to the right and one to the left. By the time the quilt is pieced, quilted and bound, all your seams are enclosed. You have to be careful to keep the fabric edges exactly together; there is little "fudge" room. In 20+ years of sewing quilt tops, I have not had one fray. But, I sure understand your question as I had sewed garments for about 30 years before quilting. Now, the shoe is on the other foot--I have a hard time sewing that 5/8" seam required on garments!