While I can see that — in theory— not ironing the fold could make a slight difference, I would want to baste the edges of the binding if I didn't iron the fold, especially if I’m going to attach it to a pieced border where I need to exactly match the quarter inch seam allowance. For me, there is not enough downside to ironing the fold to make me want to substitute in a basting step. It might be enough of an advantage on thicker battings. And probably there is a whole lot of personal preference here, just like some quilters pin everything and some don’t. I always have a nice full binding because I trim the backing and batting just about 1/4 larger than than the front.