I think I finally understand why my former very blue-collar coworkers would never leave a tip when we went out for lunch. I guess they had better information on the local businesses.
My mom was a waitress most of my life and made decent money, so I always tip 20-25% and a minimum of $5 unless I get extra shitty service. Depending on where you and your coworkers ate at, I would say they were just cheap asses.