For most rounded collar applications, this technique works well and eliminates all bulk and ridges:

Stay stitch the neck edge of both collar pieces, and interface the lower collar piece all the way to the edge. (Usually, you'd only interface to the seam line.) Sew the outer seam of the collar (the one that will be turned)with a very narrow zig zag. Trim the seam allowance as close to the zig zag stitches as you possibly can. This is a good time to go very slowly and use excellent lighting and/or a magnifier. When you turn and press the collar, nudge the seam allowance a thread's width toward the underside. I use a rounded metal spatula as a pressing aid inside of round collars. It gives a firm edge to press over, and conducts heat quickly and effectively. I keep one in my sewing box just for this purpose; it has an offset handle to make using it more finger-friendly!