echoing the other posts, generally it seems that volunteer and even clinical experience is relatively unimportant in the md/phd application process. when i applied last fall, i was interviewed by at least fifty different people (both formally and informally)... and i would guess that (at most) five asked me anything about my clinical work. furthermore, i'm pretty sure those rare instances occurred during mandatory md-only interviews (at u of iowa and umich)... so, if anecdotal evidence is your thing and you know you're gonna apply md/phd... perhaps you need no clinical experience at all! on the other hand, you might want to hedge your bet a little and at least step inside a hospital before applying to med school... ha!
in all seriousness though, you'll probably find that clinical experience helps to reaffirm or even motivates your decision to go to med school... which, despite what some people on this board would have you believe, is still part of the md/phd curriculum.
best,
aaron