I used to ask some guys to let me know so that they don't cum in my mouth, but I found that as I sucked them, I started to want to swallow, and by then my mouth was full (with his cock), plus it would have been awkward to change direction that late...so now when giving a BJ I'm simply expecting to swallow...unless the guy has some other preference and pulls out...my finding is that it's best to let males the decision on where their juice goes...that makes them happiest.