Depends on the school, some are research heavy while some are more committed to service. Volunteer work is essentially required while research can only help you by adding to your activities.
Off the top of my head, the heaviest are UCSF/UCLA/Harvard/Penn/Columbia/Michigan/Pitt and then you have schools with significant research funding (opportunities if you want them) such as UNC/UF/USC/NYU/Maryland/Minn/BU/Case/UConn/Washington/Tufts.
How important it is to Dental Schools? I have the opportunity to research in microbiology and
dental abutments and want to see how that compares, in the eyes of Dental Schools, to volunteer work.
DO IT! if you have this opportunity i would for sure go for it. applicants do enter school without 1 hour of research but this is an excellent opportunity to set your application apart especially with dental related research. speaking from experience, I felt my dental research set me apart and I was able to talk about that in length during my interviews which I felt impressed them.