TT length is more important than seat tube, and the ratio between the two varies depending on manufacturer, so the answer is, "it depends".
I'm 6'0" and currently have a 19" Orange P7 and an 18" On-One 456. I've also owned an 18" Schwinn, an 18.5" Giant and a 17" (
)) Gary Fisher, all of which have fitted well.
As a rule, I tend to regard 21" and above as for giant riders only, unless you find a manufacturer which makes a very short frame. It's worth erring on the small side, IMO, as this maximises stand-over clearance. However, this is more important if you're planning on riding very technical terrain than if you are looking to ride farm tracks and towpaths, for instance.