One of the reasons people double major is to keep two areas of study open as possibilities for graduate studies. If you haven't decided, it's not a bad idea.
The down side to double majoring is that you tend to restrict your elective options. Double majoring may prevent you from branching out within one subject area completely and discovering a subfield you really like or keep you from taking a few really interesting or practical courses. However, if you would most likely have taken the courses in your second major as electives anyway, then this is a moot point and you don't have that much to lose as a double major.
In terms of graduate school admissions, having done a double major doesn't in and of itself improve your chances for admission into a given program. Having taken and done well with a challenging course load will though.