I really prefer mine swallowed. Like TOnyi01 said, it is a sign of respect. It's a real let-down to see a guy spit it out (or worse, pull off right before I cum -- a few liars have done this even though theyve told me beforehand they do swallow.) It totally ruins it for me.
And if I'm on bottom, I tend to swallow too, have developed a taste for it just recently actually. I was gonna write I'd spit it out if I didnt really like or trust the guy very well --- but then, I wouldnt go down on him in the first place.