Some horses are Happy Juice sensitive.
Tennessee Walkers are known for being drug sensitive. I have one that can't have much of anything; half tablet of Bute will drop him to his knees and the Happy Juice he needed for a power float wouldn't phase two whiskers on a Quarter Horse
That would be my only word of caution. Let the vet know the mare has never had her teeth done, most likely never sedated and please proceed with caution
Never having had her teeth worked on, power float or manual, my guess is the vet will automatically want to sedate her.
My horses know the drill and never get sedated for the standard manual float. Love their Hearts, they stand there with that "oh crap" look and figure the quieter they are, the faster that awful thing will get taken out of their mouths
When power floats are involved, it would be sheer stupidity to NOT tranq the horse. One jerk and the drill could rub thru the roof of the mouth, where there is an artery right on the other side of the "wall" .