When a top is barebacking me. I'm pretty much expecting him to cum in my ass, and I always encourage him to do so as he fucks me. Usually seems to get them excited to do it when they know I want it. If I happen to be sucking him when he gets close, I keep workin at his cock with my mouth/throat til be blows his load.
One time, I was sucking a guy at the baths.. I was on my stomach sucking him as he was kneeled in front of me. Was sucking him good and hard for a while, then he quickly got up, moved behind me, straddled my ass on his knees and plowed his cock into my hole where he shot his load. Best of both that time!