The TouchID sensor on my iPhone 5s is a pleasure to use, but I did share the experience of fingerprint ID losing accuracy over time. Since signs seem to point to it being an issue of using the sensor different from the initial training session and diverging over time, I tried out an idea.
You can store 5 prints in TouchID, so I used all five slots: right thumb, right forefinger, left thumb, left thumb again, and right thumb again. My idea was to saturate TouchID with as many different variations on my own thumbs as I could pack in. Tips, edges, overlaps, angles, you name it.
Don't do that.
Accuracy tanked. When it could ID me, it took a long time, but more often it would fail out and I'd have to use my passcode.
I retrained it with one slot only for each print, this time trying to access the sensor like I do in casual use. Accuracy is super fast now. I'll check back later and report on whether it feels like it's deteriorating in a month or two.