If things have progressed to where his dick is in my mouth, I will manage to stop for a moment and say "Do you want to cum"? If he says yes, I take it as a big compliment and try and be as good to his dick as he's been to mine. If his dick is in my ass (which is rare, because I'm very tight, which is why I hire erotic masseurs and not escorts), I let him stay in charge and decide for himself if he wants to cum.
No matter what, if a masseur cums with me, I do my best to show my appreciation. I know he may have other clients or even boundaries that he may be deciding on the spot to go beyond. The rate we agreed on for the erotic massage includes ONLY him making me cum. Anything beyond that I consider extra--and a wonderful surprise.
And so, if it DOES go beyond that, I always tip accordingly. I always, always, always tip for extras. I would never want to be remembered as the guy who got the extras and didn't tip.