Uh, working 5-15 minutes overtime is pretty much standard no matter where you work. It's always to the benefit of the company, not you. It's an unfortunate and necessary part of having a job. Employees who leave exactly at the end of their shift are usually seen as desperate to get home and therefore not enjoying their job, it's often even frowned upon.
Everywhere I've ever worked always involved 10-20 minutes overtime. I currently officially have a 1 hour lunch break in a 9-hour day (9am - 6pm), I only get paid for 8 hours, and I usually only take a 15 min break. I usually arrive 10 mins early and leave 10 mins late. So technically I get less money per hour than what it says on my contract. So what? I'm still happy with the money I get, if they'd say that I'll get paid less but they'll also pay me for my lunch break, I'd be fine with it if I'd get the same wages at the end. Filing a lawsuit for this would be ridiculous. Some people travel hours to get to and from work, 5-15 minutes of waiting at the end of the day is insignificant.
I always think of it this way: calculate the total number of hours you work, and calculate how much you earn per hour based on your salary and your real work hours. Is that an acceptable wage? If not, get a better job, if yes, don't complain.