I'm glad this topic was resurrected. I have seen this bandied about before on lpsg. I always do; I always have. For me, it's more of the dirty talk aspect that keeps me announcing. I just always got a kick out of the 70's pornos I grew up watching. And for that matter, the cumshot announcement seems to be a mainstay of porn even in the modern day. But my main exposure was 70's adult cinema. Anyhow, even before I was sexual I saw multiple scenes where the guy says something like , "Ugggghh, I'm gonna' cum," "I'm gonna shoot," of "Here it comes, baby." So when I started getting lucky enough to get the occasional hand job, blow job, or even fuck - I would always announce, "I'm gonna cum," or some such statement.
By and large, females were pleased that I do - especially, not surprisingly, when I have gotten head. Just that 5 second warning gave them the right to decide whether to spit or swallow. In that regard, if I had a feeling I was going to shoot a particularly huge cumshot especially if it's the first time being with a woman, I'll be even more explicit. I've said, "Oh, you're gonna make me cum. You may not want me to do it in your mouth." Most of them essentially ignored my warning and swallowed as much of my load as they could, but I do remember one gal in college back in the dorms freshmen year. She gave me an awesome blow job (although I had very few bad ones in college), and I said, "I gotta' warn you, I'm gonna cum." She giggled and wisely took my advice removing my cock from her mouth and jerking me to completion. The first three jet of cum shot a foot in the air. Her giggling turned to outright laughter and she said, "Oh my G-d, I'm glad I listened to you!" I thought it was because she was glad not to have all that semen launched down her throat, but she told me it would have been a shame not to have seen that cumshot. Who knows if she was being sincere - I still think she was thinking, "Thank G-d, I would have gagged." But I like to think she was telling the truth. We hooked up several more times after that, but she swallowed only once - and the third ejaculation I had that night.