Contrary to what AggieJohn has to say, you do NOT learn in med school the things that you learn in an EMT-B class. I come from an EMS background and have been pretty shocked by how little those skills are taught in med school. I would pick an EMT to help me anyday over a 1st or 2nd year med student. EMS skills are invaluable, give you systematic ways to assess patients that really differ from the good old H&P taught in med school.
That said, you're in to med school. So if you are taking an EMT-B for any other reason than you are interested in EMS, want to go into emergency medicine, etc., it is probably more work than its worth.