I think as long as you are enjoying giving head the guy is going to like it whether he finishes in your mouth or not. If you have never tasted it and want to the next time you have sex and he blows get some on your hand and put it in your mouth so at least the taste is something you are acquainted with. The sensation of having a guy discharge in your mouth however is not something you can prepare yourself for completely. I would advise you to be in a position where you can control the depth of thrust if your doing it for the first time. You are a better judge of your own gag reflex then the guy is.