I think every Roku DVP channel I have ever used or tested has used the up arrow as the means to go "back" a screen. Not being a developer myself, I just assumed it was the default behavior. I'd say you are safe without an explicit "back" button.
Roku2 XS (13A166000325) HDMI to LG 42" LCD 1080p (42LH30)
Roku XDS (K0A073000137)
Netgear WNDR3400 (all Rokus wireless)
25 Mbps