The only time I've seen a long line at Avis MUC was arriving on an Octoberfest weekend and the counter happened to be short-handed that day with somebody out sick. Other times I've never had to wait for more than 1 or 2 people ahead of me.
Joining Avis Preferred does help with the wait time. There is usually a separate Preferred line at the counter with a very short or no wait.