On the other hand, you are letting your son know in some cases bathing is optional. Is he ready to understand that sometimes it isn't and sometimes it is? or is it better just to say Bath Time! or not say anything.
I imagine that when it's not optional, she phrases it in a more firm manner.
Yep. If it's not an option, I say, 'get in the bath.' On occasion, I say, 'are you ready for your bath?' and if the response is 'no', give a deadline - 'okay, you can finish this TV show/game/other activity, then it is bath time'.
But if it's an option, it's an option, and if it is both unnecessary and unwanted, I see no reason why I should make him.