I think it is always a good idea to have a good back-up plan. I know many very bright people who have gone to their second choice school and have had wonderful college experiences and ultimately were really grateful to have not gotten into their first choice.
What I have heard, too, though, is that college admissions people get tired of looking at applications that are too slick and not personal enough. It's just like applying for a job; you don't just say here are my qualiications. You say "here is why I want to be at your institution; this is why I am a good fit." In my day, person might apply to 3 schools. Now many kids are applying to 10 or more and the applications lose that personal touch. Other factors being similar, they will pick the one who really wants to be at their school.
