First, those applications are pre-interview right? I always thought that interviews went both ways, not only do they interview you to see if you would be a good fit for their school, but you also determine if their school is a good fit for you. After all you are going to be there for the next 4 years! And if you determine the school is not right for you, I am assuming you would not reapply there.
Secondly, nobody is saying to hold out until you get into your 'top choice' school. The majority of applicants apply to more than several schools, and there are many factors that dictate what particular schools you choose to apply to. But if for some reason you find that one of those schools doesn't seem to suit you than the only mature thing to do is to reject them.
Third, I am sure that the OP realizes the risk involved with turning down an acceptance to Medical School - as does everybody else on this site. With that being said if the OP truly feels she could make herself a better applicant for the next cycle and increase her chances at placing at a school where she will be happier for the next 4 years more power to her. I am pretty certain adcom commitees are smart enough to realize how big a decision this is, and will not hold back the OP just because she had the maturity to not jump at the first offer she gets. I would also think that the drive to better yourself and work towards making yourself a better applicant shows your motivation and determination to succeed at accomplishing your goals.
Personally, I work very hard to accomplish my goals and if I where in this situation I would not 'settle' for a school that I felt I would not be happy at; unless, I felt there was nothing else that I could do and this would be the only route at accomplishing the greater goal of becoming a doctor. But in this particular situation the OP feels she can improve her situation in terms of competetiveness so I feel she should give it a shot.