Or require standard, non-magical, non-modified coins. Dented and scraped from normal use? Fine. Message carved on it? Nope! Spell won't work. Carefully dented in a particular pattern by precision-dropping daggers on it? Nice try. Magic Mouth (or any other spell) on the coin? Yeah. "No." The intent is to use it for commerce. If your goal is not commerce, Waukeen won't permit you to abuse her spell.
I don't think I've had any homebrew come back to bite me in the butt, but that's probably because my game simply didn't last long enough.