Hey Vench,
Many times, or I guess in most cases when it comes to pro productions, you let a dedicated mastering studio do the job. Few engineers/producers master their own pruductions. But I don't know, maybe I'm wrong.
There are several reasons for this, but most of all because mixing and mastering are two completely separate things. I know a pro engineer/producer who owns a comercial studio facility which includes two recording studios and a mstering studio. He has recorded several records that went gold, and mastered several records that went platina. Anyway, he almost never master his own productions. He said the whole point is to have a guy with a different set of ears do the job. Of course, the guy has to be skilled, or the production would get ruined.
Have you ever had a 16 hours recording session, where in the end your ears get so tired that you think everything sounds like crap? Well, you get the idea... it's good having someone else with a different perspective master your records.
My two cents,
DD