You need to sew "scant" in order for the turn-of-the-cloth in the seam. The "scant" part is how far from the raw edge your needle and thread are in order to achieve an "exact" 1/4-inch seam when you have ironed the seamed piece flat.
The degree of "scant" varies depending mostly on the size of the thread you are using for piecing. I just switched from a standard 50/3-ply thread to Aurifil 50/2-ply, which is a visibly thinner thread, and found that I can sew much closer to 1/4" from the raw edge to achieve an exact 1/4" after ironing. It's amazing how much thread size affects the turn-of-the-cloth in a seam!