I wouldn’t want to know. If she tells me I’ll die in a car accident, then I’ll spend the rest of my life afraid of cars, since I’d never know which one might hit me.
Even assuming she’d tell me exactly how and when it would happen, I wouldn’t want to know. I would live dreading that day, thinking about it… I prefer blissful ignorance.
And of course you’d have no power to change that future. She’s a witch with magical powers! Any prophecy from a witch or gypsy always comes true, in the movies. So if I’d desperately try to avoid cars for the rest of my life, living secluded in a forest cabin, someday a plane carrying a shipment of Toyotas would explode overhead, and I’d be squashed flat by half of a flaming Corolla.
I think that, given enough time, most people would. This idea is based on something that Edgar Allen Poe suggested, and it’s called… The Imp of the Perverse. In a nutshell, you’d do it because it was the one thing that you know damn well you don’t want to/shouldn’t do.