I just finished my b.s. in biology with a chem minor. It makes you more competetive to many schools, partially because you've demonstrated commitment in staying the course for four years, also, it should make you a more well-rounded student and person. It's not necessary, but it's helpful. Also, it doesn't really matter what your major is as long as you complete your desired school's prereqs, but bio or chem majors are t he easiest to work around those requirements (usually)