Depending on the frame design, intended use and your body you can vary sizing quite a bit. e.g. compare a road bike with the same persons tt bike or tourer.
I'm really can't string sentences together now so I'm gonna leave it a while.. ride what feels right after "trying" lots of different options ("trying" isn't "looking at").
Who? Me? Nah.. I keep buying the wrong size frames.. don't ask me for advice! :)
Sheldon goes into loads more detail: http://www.sheldonbrown.com/frame-sizing.html
Depending on the frame design, intended use and your body you can vary sizing quite a bit. e.g. compare a road bike with the same persons tt bike or tourer.
I'm really can't string sentences together now so I'm gonna leave it a while.. ride what feels right after "trying" lots of different options ("trying" isn't "looking at").