You're kissing them, I'd say the etiquette is to tell them. Unless this is a drunken pash, in which case you run away and avoid drinking alcohol in their presence.
You could do the "here, have some gum, no HAVE SOME GUM" whereby anyone with an IQ 80+ will realise they have bad breath and gratefully accept it. It's a little less tactful than the simple telling them though if they're in any way intelligent.
Speaking of gum...i brought the cutest pack of gum from an asian store the other day...they are pink, Lotte. So nice but only last about 10 minutes. I brought them cos of the pink packaging...sucker.
