The letter of intent is not legally binding. Nor is it unethical to back out afterwards. There are all sorts of considerations once you have a choice between schools. I sent letters of intent to all of my schools. Had I gotten into one, it would have been my top choice, as stated in my letters. However, once I got into several schools, I had a choice to make, and a lot of considerations to keep in mind. My "top choice" quickly moved down when I was waitlisted. Other choices moved up because of acceptance without waitlist. Even after I had multiple acceptances, things changed because of scholarships offered, one school offered in state status, another offered funding, etc. I would have gone to any one of them had they been my only choice. I ended up turning down my top choice because they decided to accept me off of their waitlist in August, 2 weeks before classes started, because another school offered me scholarships and in state residency, etc.
Now I DID do the ethical thing concerning multiple acceptances. Once you get multiple acceptances, you have up to 2 weeks to make a decision and go with one. You get 2 weeks for each subsequent offer if they come up. I turned all down as soon as I made my choice amongst the schools. That way they could offer the spot to other students.
People need to get off their soap boxes about this. Once you're in med school, and getting ready for the match, you'll see how institutions will tell you they will definately rank you very high, then after the match, you're left to scramble. It has happened to some good friends of mine. They will lie to you, go behind your back, etc. All I'm saying is be moral, but watch out for yourself too. You may find yourself with out any med school acceptances, or just as bad, without a residency.