Even if it wasn't an optional bath, there really wasn't anything wrong with it. I know I've phrased mostly nonoptional things in the form of a question to my sons before, just because that's how it came out. If they said "No," I shrugged and said, "Tough!" with a grin.
In a slightly related note, I once had a perfect stranger scold me in the grocery store over issuing (minor) reprimands/instructions to the boys with a "please" attached. ("DS1, please put that back; we don't need cookies this week." "DS2, please stop badgering your brother.")
"You're the boss!" she told.
Well, yes, but with minor stuff there's no reason not to use "please" with it; for the first comment, anyway. I want them to use the word; I should as well.