Unless you are 100% sure that you and your roomate...
1. Have similar sleep/study schedules = example: I skipped a ton of class, slept a lot during the day, studied a lot until 5AM obviously with lights on, music on, etc.
2. Have similar social situations = relationship status, views on drinking, views on partying, hosting parties, etc.
3. Have similar hygiene standards = cleaning the place, loud music, etc.
4. Have separate phone lines or each have a cell phone
... do NOT get a roomate.
Med school is very stressul and you do not have the time to deal with roomates that don't see eye-to-eye with you. Contrary to popular belief, not all med students are your run-of-mill study 6 hours/day, quiet, no partying, bedtime at 10PM, neat, clean people.
And the consequences are more severe than in undergrad. A bad roomate could negatively impact your performance.
You have enough bullshyt to deal with in school- your home shoud be a place where you can kick back, relax, and escape into your own world.