There has recently been a lot of debate on this topic since Blue Hors Matine and her WEG gold.
The general concencus on Blue Hors Matine is that she is a very sensitive mare and is not swishing her tail out of discomfort, but instead from being "ticklish" and working through her body.
We have to remember that horses naturally swish their tails. Why else would many breed circuits nerve, break and position tails. They just don't want them to move like they would naturally.
If you watch two stallions fighting, they are both going to be absolutely collected, just as we want a dressage horse. And their tails are going to be wringing around. I have no idea why this happens, but it seems to somenow be a product of collection.
Another explanation is the nearly constant aids and half halts that dressage riders are giving their horses. Although on the surface everything looks calm, in order to keep the horse moving perfectly there is constant adjustment and readjustment every stride. At the stage in training where these dressage horses are ridden to perfection, they are also very very sensitive and so this constant aiding by the rider can be met by some ticklishness that will cause tail swishing.
Yet another explanation is maybe it is just a concentration thing. When you are working very hard to think and concentrate you may chew your lip, pull on your ear, etc.. This could be the same thing with horses is that when they are at that high a level of concentration they might just wring their tails out of habit.
A final explanation is that of discomfort. Some horses either as a result of natural lameness, lameness from poor conditioning/use of gadgets or rolkur will be sore and so may wring their tails because it just plain hurts. This is usually accompanied by a sour expression, pinned ears and other disobedience (unless it's been beaten out of them).
So yeah, it depends on the horse, the situation and the level of competition. If I saw a training/first level horse with their tail wringing I would assume that the horse is in discomfort. Where as a horse like Matine is just working well and hard.