It's dangerous for both parties. But IMHO there is no protocol as far as asking permission and it is good form to do your fair share of the work - particularly on a century.
Much like Stapfam said, if someone doesn't like you drafting - they will find a way to get you off their wheel. However - unlike WL said, and again IMHO 6" off a persons wheel is too close except if you are on a track with good & known surface conditions. I did a 64 mile ride today and did a lot of drafting, on back roads with broken pavement, road kill, ups & downs there is just too much going on not to have a lot of room to bail out or adjust. About a wheel diameter is what a I use as a guide. There is plenty of suction that far away - particularly behind a big rider.