In my opinion, the priority list goes: good grades/scores, clinical experience, research work/volunteering/notable EC. I think how important it is also depends on the med schools you're applying to and how many other EC's you have in general. Some schools place a heavy emphasis on community involvement and look for non-medical volunteering, other schools put more emphasis on research, etc. And if you have a lot of EC's, and some are already non medical (like competitive sports), I think you'd be ok.
I think if it's something you want to do, and you find a project you really love doing and spending time on, then by all means go for it! But I don't think there's a need to join a volunteering club and do it once and awhile just to put it on your resume at the expense of doing an EC that you thoroughly enjoy.
Just my two cents. I personally didn't have any non medical volunteering at all, but did A LOT of clinical work, and I ended up with some good acceptances. (Granted, many were at research heavy schools)