More formal because they happen to be on evening shirts? I'm seriously not getting it. Maybe I too am of the mind that buttondown and double cuffs are simply another combination one can come up with. After all, no one seems to complain about point collars being on casual shirts or barrel cuffs being on dress shirts with point collars.
Hope I don't come off as pretentious, but I'm not eternally bound by all these endless rules on dress. Like Jason, if I see something and think it'll look good, I do it. I do know what to avoid to give off the wrong impression subconsciously in others' minds depending on situation. Point is, this just doesn't seem like one of those things.