Most of my girlfriends didn't care if I told them or not, as they could always "tell". However a general "I'm close" or "I'm going to cum" or anything like that is generally okay. If you still feel stupid after saying that, just ask her beforehand how she'd like to be warned. Some girls like to be tapped on the shoulder, some girls don't like anything at all.
Again, no girl I've ever been with really minded me holding their head, just not pushing or forcing really hard. If you want them to go down further, just tell them. I am sure most girls don't mind at least trying to go down as far as they can, if they do then deal with it, they're sucking your dick, they kinda make the rules.
I never minded kissing a girl after a blowjob.
