A good rule of thumb, I imagine, is this: Don't be any stupider than you have to be.
And yet it is interesting: As smart as anyone might be, there is always room for more stupidity.
Soen Nakagawa Roshi, my Zen teacher's teacher, used to encourage his students to whack him on the head. He seemed to know a thing or two about stupidity ... most notably that there was always room for more.
It's all enough to make even a stupid man reflect.