Oh fuck yes! I absolutely love it when my wife does that. It not only makes it easier to suck the nipples of large naturals that sag down a bit, but it's great for making them look more "perky" (for those of you that don't like them saggy). It also helps women who are more "silicone" or, worse yet, "saline" based look more natural too -- it looks like the bra is shaping them and not the wafer inside.
I love it too when my wife wears a nice bra during sex. She has great tits and great bras.