It depends. I've sifted through pretty much all of the existing threads on stethoscopes and it sounds like there might be some schools that prefer certain ones for their vet students, but in general it's a good idea to get one so you are familiar with the instrument and using it. Littmann is the go-to brand and I LOVE mine. When it comes to model, it's a balancing act between personal preferences, budget, and intended use. I work with equine and large animal vets, so for my budget it made more sense to stick with something that very well might get trashed, so I scoured these threads and ended up getting the 3M Littmann Classic III. I love it and I know down the road if it takes a beating in the field I can invest in a new one or I can upgrade based on my plans and interests and/or the school I attend. Does that make sense?
Good luck and happy stethoscope shopping!