Indeed cats can get stressed out going to the vet - so do people going to doctor's appointments. (Who can blame them.)
Mine used to shed alot going to the vets when I first got them but since they have been going there alot over the year and a half the shedded had settles a little.
I haven't done anything to try and stop it either (though I do spray their carriers with Feliway before they go in). *Shrugs*. Though I still get some fur from Kuce when she sees the vet more than Sphinx (Then he's been in more for his health problems). They settle down once in the car and back home.