In a real relationship, that is a definite no-no guys as many trans girls interpret it as a sign of rejection.
If you care about her, it wont bother you.
If you don't though, perhaps better not to try.
Looking up and seeing that sexy womanly form all perfumed up to the 'Nines' is superb but you cant chicken out once that mouthful of salty musky goo comes out.
If you've done it in real, you'll know that it is much harder than shown and takes a while to acquire a taste.
I assume that most men in trans porn spit right after the camera moves away.