If you're taking a survey, put my vote on the "grumpy" rather than "ache-y" side.
My mare is grumpy. She doesn't like dogs, cats, sometimes people; and she LOATHES other horses. Swishing her tail is her way of showing her displeasure, of protesting (when she has had enough of being messed with), of saying "back-off."
Interesting side story: For the past 12 days, I have been treating my mare twice a day for cellulitis. Each visit includes dousing with cold water, dosing with medications, flushing the wound site, walking a mile, and dousing with cold water again before I leave. It takes me about an hour each time. She's absolutely fed-up with being messed with, at this point, and I can't say that I blame her. When the infection was at its worst, she was in a lot of pain, but she didn't swish her tail at all.
In fact, I could tell she was feeling more normal when she began swishing her tail again!
Anyway, I noticed in your original post that you said your gelding tends to be "grouchy" and added that he tail-swishes when he backs-up (which is one of a horse's least favorite things to do). So I'm thinking he's like my mare -- a curmudgeon who shows his displeasure by tail-swishing. Just my two-cents.