Currently, key binding for turning on/off input methods (switching Roman <--> (Japanese) input methods) is "Alt+Space" or "Alt+Zenkaku/Hankaku". (FYI: Zenkaku/Hankaku is a key which Japanese keyboard has.)
Alt+Space is good.
Alt+Zenkaku/Hankaku is required to change in Japanese community. Please no-midifiers Zenkaku/Hankaku, not Alt+Zenkaku/Hankaku. Currently, Zenkaku/Hankaku itself has no function, so, it is reasonable to make no-modifiers Zenkaku/Hankaku turn on/off.
Actually, old versions of Windows used Alt+Zenkaku/Hankaku to turn on/off input methods, but, current version of Windows uses no-modifiers Zenkaku/Hankaku.
cf: JPBE.net IM relative forum thread
http://www.jpbe.net/forum/viewtopic.php?p=7763#7763
Haiku: r25124+
SHINTA (enhancement No.2)