okay, you asked so i'll share...
if you've ever tried to use rayon bias tape for a facing, you'll know how tricky it is. it shifts, it frays, it's generally uncooperative. before my portrait blouse, i had done it twice: the first time was a bloody mess, the second time took for-ev-er! and still didn't look that great. so i was searching my pile of scraps for a lightweight cotton or basically anything that would work as bias facing. nothing. okay, fine. [deep breath] i'll use self fabric.
i didn't take pics when i constructed the blouse, but i used scraps to show you my method.
leave the full 5/8" seam allowance on the neckline of the blouse and stay stitch at 1/2" (or just inside the SA) directionally from the shoulder down to center front, then shoulder to center back. make sure you overlap those last few stitches at CF and CB. stay stitching makes a world of difference, trust me!
cut your bias tape to 1 1/4" in width, then serge one edge with 1/4" wide serging. if you are living in a cave and don't have a serger, take your bias tape and press one side in by 1/4".
line up the edge of the blouse with the edge of the non-serged (or pressed) edge of bias tape and sew at 5/8". don't bother pinning the bias tape all the way around first, just go slow and keep adjusting the bias tape as you go around curves. don't stretch the bias tape or you will end up with a puckered seam.
once it is attached, trim the seam allowances down to 1/4".
press the seam flat first in order to shrink back any stretching that may have occurred and to eliminate any wavy-ness at the seam.
now lift the bias tape out flat and press the seam open from both sides. this will give you an good clean edge.
press in the serged edge of the bias tape using the serging as a guide, or re-press the 1/4". seriously though, get a serger.
now turn the bias facing into place and press. pin as little as possible perpendicular to the bias tape. excessive pinning or pinning parallel to the bias tape can distort the seam and if you have adequately pressed along the way, you won't need many pins.
finally, topstitch at a scant 1/4". i find that if i move my needle to the left position i can get 1/4" by lining up the right edge with the edge of the opening on my presser foot. i tried using my 1/4" piecing foot as a guide, but it was just a smidge too wide.
now you have a perfectly bias-faced edge!
this may seem like a lot of little steps that take too long, but in reality it goes very fast. if you half-ass or skip any of the pressing steps, it will take much longer and not look as nice. true story.