I have to say I crammed for most of my exams in undergrad in remember very little of the specifics. The value of your undergrad courses is more becoming familiar with the subjects, learning how to study, learning how to think scientifically, learning how to look up scientific journal articles. Knowing the basic biology is important, they gloss over a lot of basic biochem and cell bio assuming that you know it. But you'll pick that stuff up pretty easily just by seeing it again and again in your courses. As others have said, physiology, anatomy, immuno, biochem, genetics are good classes to have a pretty good handle on so that the subjects are more familiar in vet school, but you mostly won't remember the details and don't need to.