My road racing bike (Ridley Noah XS) has an effective top tube length of 52cm. With a 100cm stem it fits me rather well at a 10cm saddle-bar-drop.

My TT bike (Slice Hi-Mod 51) has a top tube length of 50.5cm. I use a one-piece aerobar setup from Vision for which I cannot remember the specs offhand, but the saddle-bar drop is 13cm or more. I am able to extend the seat post higher and get a more compact forward position. This gets me a slightly better CdA than my old time trial frame which was larger. The results have been around 1mph increase in TT speeds (at same power) since I got the new frame.

Basically, what I am saying is that my anecdotal evidence backs up the common practice of going "one size smaller".