i wouldnt call anyone other than Icehawk Master or Sir,
we rarely use them anyway, if we are roleplaying and im say a schoolgirl or maid for eg then it's Sir, its slave, or being 'kidnapped and thrown in the dungeon' type play then its Master

i think Master is sometimes considered the correct title over Sir for those in a Master/slave relationship but really its down to the individuals

i think it can sometimes depend which part of the world you're from too ive noticed that in some countrys or regions of countrys terms like Sir and Maam are regular titles outside of D/s over here they're not except for school teachers and such but perhaps if i were from a country where it was more commonplace to call people Sir then that wouldnt have the same authorative connatation as Master.