Some people will tell you that it doesn't matter, and some people are grasping onto their double major for dear life. In all honesty, every little bit helps, but it is hard to tell you what the dual-major/GPA trade-off is. Is it worth another major if your GPA drops by .5? Almost certainly not. Is is worth another major if your GPA doesn't change at all, you like the subject, and you would just as soon spend your extra time studying something in addition to science? Sure.
I think that I may have just labeled myself a nerd in the previous paragraph, but I am graduating in December with dual majors in Economics and Biology. If I was doing it again, I would NOT double major. The Economics degree has certainly cost me in the GPA and time section that I wish I had back now. On the other had, I sincerely enjoy economics and might consider a PhD if the MD doesn't work out. I truly enjoyed the breadth of education that an Econ major gave me, but there are other ways to be broad.
These threads are only useful because other people can help you see angles you might not have, trade-offs and incentives to consider that you might not have come up with yourself....but nobody can tell you what's best for you.
That's all my opinion folks.