The grand finale of a blowjob is definitely the cumshot. At this point, you should let your man decide how to deliver his seed. For you, it is a gift and you must accept regardless of the way it is deposited.
Each of the ways will bring you excitement. If he cums in your mouth, you can enjoy the taste of semen, lick the last drops from his penis and clean him up. If he decides to be messy instead and come on you - even better. You can live your dream and act like your favourite pornstar while ropes of sperm are covering your face.
Don't wipe it off, though - use your finger and eat it off your face, it's delicious.