All of our horses of course have been done many times before - they are all on a once a year maintenance check-up, which certainly makes it easier on both the dentist and the horses.
I find it hard to understand why people don't bother to have horses' teeth done regularly - we've had several arrive here who have not been done for years, and have developed sores or even ulcers from sharp teeth.
Even if they tolerate being ridden, or are ridden bitless, it must be horrible for them to have sharp points digging into their cheeks every time they chew :-(
Interestingly, these horses have not usually shown too many obvious problems chewing or being bitted, which makes you wonder how bad things have to get before there is an "obvious" problem.
The other issue of course is that once teeth have been neglected for a few years, it is a much longer and more difficult process for the dentist to sort them out, and more uncomfortable for the horse. By contrast, our own horses are all happy to done by the dentist, and even seem to enjoy the process.
PS: I am a complete hypocrite of course, because although I get the horses regularly done by the dentists, I am far too frightened to go anywhere near one myself :-0