Many of them constantly tune their boots, but many also ski their boots flat.
As has been noted elsewhere on Epic, WC skiers will often improve their performance with changes, but decide it doesn't "feel right". And, they are far more able to overcome the compensation requirements than less well-tuned athletes like most of us...
Also, according to a former WC boot tech that gave a presentation I attended recently, they often have their different boots tweaked differently for the different events, and most have at least 3 pair of boots. An O-stance (thanks, spyderjon!) for slalom, and flat or a bit in for speed events.
But, to be clear, it's very, very individual at that level. Similar to Tiger Woods being able to tell a gram or so difference in the weight of a golf club.