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New SDN Article! High Yield: One Year of "Research for the Rest of Us"
If you've got spare slots, yes of course you should add them. It's better than adcoms thinking you didn't work hard 
I didn't count old high school jobs but I included my serving positions through college which my interviewers (at least at UAB) were really interested in so I think it was a big plus. It shows you have people skills and work through schooling. Definitely up to you though!