While doing my autofocus fine tuning with all of my lenses last year, I came across a dilemma with my 50mm f/1.8 lens. I found that the AF fine tuning adjustment for it was consistently high on all three of my camera bodies. The range was 11 to 16 as you can see in the attached test result files. I thought long and hard about these test results before deciding to keep the lens and not have it repaired.
The first thing I did was to look at the graph of the test data for each camera body. Each of the tests indicated the Result Confidence was excellent. In other words, the data did not have a wide spread along the final plotted curve. The tests also converged on a final set point with a reasonable number of test shots. Finally, the curve for each body rose to a high point and then dropped off to the right of that point; the data showed a clear inflection point in each case that was not close to the edge of the set point adjustment range. All of those observations gave me confidence that the lens AF would perform well on each body even at low apertures.
My next consideration was how much I used the lens and how critical AF would be for me when using it. The widest aperture of the lens is f/1.8 which makes it good for low light situations. That is also where the accuracy of the AF system is most critical because of the low depth of field with wide apertures. Having a high AF fine tuning setting did not give me comfort knowing that I would want to use the lens in low light, but I also knew that I would seldom use the lens. I actually bought the 50mm lens as a backup to my 24-70mm f/2.8. The test results for my 24-70mm showed an AF fine tuning range of -3 to +2. I liked those results. Thus, I decided to keep the 50mm f/1.8 as a backup to my 24-70mm and not have it repaired. While a repair might bring the AF fine tuning adjustment down, it would cost me over $200 for the repair, and the results might not be much better.