I wah with my right foot, because like you, I find it easier to control the little nuances. So I guess that'd be my picking hand side.

HOWEVER, I might not be a very good person to set a standard by...I am a lefty who plays guitar right-handed. I just find the right foot feels more comfortable on the wah.

But there's another thing you should probably keep in mind, too - balance. When I play, I tend to have my legs spread fairly far apart, with my right foot forward. This way my right upper leg is against the guitar's body and pushing it outward a little, which I find makes it easier to fret. So with my right foot already forward, it's just natural that I'd use it for the wah. In order to keep my balance and play the wah with my left foot, my right would have to be somewhere past my effects. :p