Black Tie???

I think if you are in doubt, you can do 'Black Tie OPTIONAL'. It sets the tone of your wedding but it doesnt force people to go out and rent a tux.

FH and I are doing this b/c I have some people on my side (and even a few on his) who can be dressed a little less than appropriate (to say the least). We don't want to impose any added expenses onto anyone but we do want people to know that it is formal affair.

Usually depends on the RH, how formal your BMs dresses will be and the time of day, usually evening weddings.

You can give your guests a choice if you state Black Tie Optional. So that they know you prefer it to be more formal, but leave it to their discretion.