Hmmm. I usually spit, not because of the taste, but because swallowing makes me feel like I have a cold (another occasion when you have lots of slimy stuff in my throat). Unless it's impractical, like once when I did it on a train. : )

It seems to me that the taste is often romanticized in fic (people are always "oh, you taste so good!"). But I guess that's just part of the general "fantasy" aspect of it. And I do find it hot when people swallow in fic, and I guess that's partly because psychologically it implies an acceptance of the other person, and partly (like Vulgarweed said) because spitting in itself isn't very erotic.