It makes no difference whatsoever, except to the extent that you run out of adjustment room for the saddle fore-and-aft. For example, if the seat tube angle is very steep, you might not be able to put the saddle back far enough for *you* and your riding style (because you run out of room on the saddle rail), or vice versa. Other than that, a few degrees more or less makes no difference at all. You don't sit on the seat tube, you sit on the saddle.