I had an interesting snippet of conversation with one of my lovely trainees, Amy, today. We were discussing "eye pop" (using Levels, of course!), and Amy asked me if I ever found it necessary to selectively sharpen eyes.
The answer is no, hardly ever. I've probably only done it once or twice in the last year.
For one thing, sharpening is not the best way to make eyes pop. There are invariably better and more natural ways to do it. I don't think eyes should get any more or less sharpening than the rest of a photo.
But that's the thing. When sharpening is done properly, it automatically applies most strongly to the areas of best focus. And what is the usual focal point of a portrait? The eyes, of course. So when I sharpen a portrait, I find that as long as I use the Threshold slider correctly in USM, the eyes get the main "hit" of sharpening, without having to trouble myself with selecting or masking.