Personally I've never had a problem with this. If the girl offered CIM I don't feel the need to ask if its on offer that day. If she says otherwise then fair enough. As for telling her when you are about to cum, as others have said, I think they know.
A girl I'd seen a couple of times offered CIM but I hadn't tried, instead opting for Facial and cumming over her tits. On my third visit she actually said "I think you want to cum in my mouth" she was right. Since then It's never been a problem; if I'm face fucking she knows it's coming; if I'm wanking over her face she usually opens her mouth anyway and, like yesterday, she was giving me oral and wanking me and directed my load over her face and Into her mouth.
Had a girl recently who pulled away when I came l but I think that was more to do with how she was feeling rather than I didn't tell her. She knew by my moans and groans I was about to unload but I think it was too much for her first of the day.
Some will ask that you tell them, but generally I've found they go with the flow.