A majority of schools ask for your final transcript once you matriculate only to prove that you successfully graduated with the degree you claimed you would get before matriculation (your final degree is officially recorded on your final transcript). Some actually care that you do 'well' but will likely say so in your acceptance letter. The idea that a school, no matter how prestigious or competitive, is going to kick a student out upon matriculation when they find our he or she got a 2.5 senior year is a neurotic myth. If you fail courses, you likely won't be able to graduate when you normally would and wouldn't be able to go to medical school for that reason.
That being said, I know of at least one school (UCSD) that explicitly asks for your grades when you go to interview. However, that was the only school that seemed to care even the slightest about my then-current performance.
Ultimately, the chance of senior year grades meaning anything is unlikely but it is definitely possible that it can hurt you if you truly slack off the the point of doing well below your submitted-GPA average.