Here is my 0.02 cents: I am not sure how anyone earns the title Master Printer, however, with that being stated here are my following points of view about this. If a person just learns some basic techniques to print one particular style, does not experiment very much, yet has been printing the same way for 20 years they have mastered a certain technique, but I would not call this person a "Master Printer."
There are a few ways that people earn this reputation:
1) The person has been printing commercially or in a commercial lab and has built up a reputation in that particular community, and everyone goes to that person to have their prints printed. One example is the person, who started Gamma Lab in SF. and is an incredible printer. What makes him so incredible is that he really knows his tools and how he manipulates how to use the various filters during the printing process. He might need to use a grade five filter one part of the print, a grade 1 filter on another part, etc . . .
2) A printer who has overtime picked up well named clients such as Magnum photographers, and by reputation all the magnum photographers go to that one person to start printing for them. I forgot her name, but she was living in NY printing for a few of the magnum clients, then moved to SF. Her clientele has dropped a little, however, she still has a following based on her reputation of the services that she offers such as selective bleaching and selenium toning.
3) Another way is that someone has learned how to print from several well known photographers who have built up that reputation and passed it on to an apprentice. Ansel Adams was notorious for having many well named people who assisted him, such as Dorothea Lange, and there were many more.
I also know that there are many people, who have had their own experiences learning how to print and have been printing for numerous years, more for themselves, yet who also have never developed a reputation for themselves of any kind, but kind of learned on their own.
I have been printing for the last 30 years. However, I do not consider that I really learned how to print until 1990. I mark this as the beginning of my photographic, both as a photographer and printer. Prior to this I was just doing the same thing in the darkroom. Setting up my chemicals printing 8x10 RC prints.
In 1990 I felt like I woke up and realized how much more one can really accomplish in the darkroom just by varying your printing technique. I started to take classes with one photographer learned a little, then between 1990 and 1995 I began seek out people who really knew how to print; Frank Espada was one of my greatest instructors, who was Eugene Smith's assistant. I was also able to take a brief one or two hour printing session with Brett Weston, another great influence, who I was able to spend some time with is Roger Minick. I also started to learn how to do alternative processes such Daguerrotypes, platinum palladium printing, and anyone else I could seek, who would be willing to teach me what they knew.
So now when I make the master or gallery exhibition prints, I spend hours testing the image. I start my developer at 68 degrees, and start with graded paper test strips. First, I determine what grade this image is best suited for. grade 2 or 3 and sometimes grade 4. Once I sort have determined a a grade, I will spend hours testing the image at different dilutions and temperatures of developers. I will also add chemicals to my developer. once I figured out a combination with this, i will sometimes test the image using a two developer bath, usually selectol soft and dektol. Again, testing the image at various development times and at different temperatures. Once I finished with all of this, I will begin with a full sheet of paper, burn areas where I want to burn, but never dodge. I print for the shadow, and burn down the highlights and midtones.
From there is becomes a process of selectively bleaching prints or deciding if the whole print needs to bleach, and then toning the image. I will also experiment with images, different types of papers, developer, and developer combinations, I will also experiment with various additives that enhance certain things when developing a print. Once I am done testing print, I will make various prints from my notes and then decide which combination I like best. I will make several full sheet test prints for various purposes, but to mostly test with the variations I can enhance the image by toning.
After spending many hours, especially in the beginning years, you eventually just look at the images and know what kind of developer combination will be most effective along with your knowledge of toners and how they will effect the particular paper and image that you are using. It becomes second nature sometimes getting the results you want from all of your previous experience and the knowledge you have gained from the people who have taught you.
In conclusion, I would consider someone a master printer, where printing has become second nature to them, where they can look at an image on a negative and think of a few different ways to print that image that would enhance it to make it more intriguing or powerful.
I hope that helps.