Frame size is also not an exact science. Some people prefer small, zippy, and light as possible, where as others (like me) go with big as possible so that it rides like a big comfortable cadillac. So unless you already know what style u like, which it takes a few years of biking to really know, then you are leaving it up to the LBS's discretion anyways, which usually means (a) they will size it on the smallish side cuz most LBS's I've been to think with racing parameters in mind, or (b) more cynically, they'll recommend whatever size is in stock like swduncan says.

The sizing write-up that works best for me is the one at rivendell's:

http://www.rivbike.com/article/bike_...g_a_frame_size