Mastering is an art-form in itself. To me it splits up in the artistic/creative mastering part and the technical mastering part:
For the technical part (preparing masters for different distribution media , like Vinyl, iTunes, CD etc.) I rely on partners that have the day-in-day-out experience to deliver professional results.
As for the artistic/creative mastering part I always deliver to my clients fully mastered versions (...as I mix into a mastering chain that lets me control balances and frequency content up to the last second of a production.
I also deliver a non-mastered version of my mixes , so clients can have mastered by outside mastering engineer or do mastering-shootouts to get different mastering-results to compare with.
If I do mastering for outside productions, I only commit to projects that interest me in an artistic/creative context. It really depends on the project.
So, please feel free to contact me at anytime for a mastering-request/quote for your latest project