I do once/year just before I start riding again in spring. I used vets for several years and was satisfied with their job, but I gave a try to the eq dentist this year (who does very fancy horses at my trainer's place, so she recommended him to me). I was very pleased with the way he worked. Definitely way more gentle and polite with the horse than either vet I used. However the disadvantage of using a dentist (for me at least) that I have to combine dentist and vet to give the sedation. But I plan on calling him out again next year. :)
"Life isn't about waiting for the storm to pass: it's about learning to dance in the rain..."
"When we are no longer able to change a situation - we are challenged to change ourselves."
"How people treat you is their karma; how you react is yours."