Mares rarely get canines and if they do they are usually small and immature. Whether they are large or small, grinding them to the gum line is dangerous and can create a pathway for infection to pass to the root of the tooth. Since canines sit ahead of the bit they can't cause a problem with bitting when the bridle and bit are properly fitted. If the mares problems are in her mouth and tooth related it's much more likely the problem is in the back of the mouth around the outside edges of the upper last molars. I'd be wary of any dental professional who spends too much time on the canines since, in my experience, they rarely cause trouble but are easy to work on.
My vet shapes them up a bit, if only to help the horse's whose owners bonk those canines with the bit on the way out of the mouth. Definitely does not do anything remotely like take them down to the gum line, at all- just lowers their profile, per se.
I wondered, as I'd never hard of it before, either.
Is it possible that they could be poking her tongue when she is being ridden? Those canines are SHARP and small, and her mouth is very low-palated (SP?) with a thick tongue.
She is getting very fussy, where she wasn't before once we found this bit that she liked (a happy mouth mullen loose ring).
It is almost as if she is more fussy if you haveher on a light contact in the warm-up but once you have a slightly stronger (but still elastic) contact for the actual work, she settles in much better.
I tried to have a look today and she was NOT letting me get near her mouth, although she'll open her mouth for the bit. Very strange.
My 17 year old pony just had her canines removed when she had her teeth done a few months ago. One had broken off, and the other one looked like it was starting to get infected. My vet said that she usually leaves them alone unless they are causing issues- she felt like hers were, since the one was broken off and the other was looking slightly infected.
I would talk to your vet and see what they say. If nothing else, if you think something in her mouth is bothering her, maybe it would be worth it to have the vet out to examine her mouth and make sure that everything is ok.