Doesn't seem to me that it would matter a lot. You definitely may observe more interesting cases at a teaching hospital, but other than that, clinical volunteering is clinical volunteering. Go wherever you know you'd get the most valuable experience (how much can a volunteer do? Shadowing opportunities? time commitment? Leadership opportunities?)
Personally, I got the BEST hands-on clinical volunteering experience at a non-teaching hospital, whereas my university's teaching hospital volunteering totally sucks and consists mostly of wandering the halls and filing papers.