It depends. I'm 5' 7.25" and prefer a 52cm. My cycling inseam (ground to crotch, while wearing bike shorts) is about 32" However I prefer a shorter top tube, but since I don't want to get a custom bike or buy a 50cm, I use a shorter stem (90mm).

I've heard a very important part of bike fit is the top tube length. Seat tube doesn't matter as much because you can easily adjust the seatpost. However to change reach, you would have to mess with different stems.