Q Do you think it is a waste of time to make a complaint about service?
A It is absolutely not a waste of time. Complaining about bad service is appropriate right = approriate for customers.
Cost for = The cost of  service is included on the bill. Of course, it is wrong to complain the serve=service with disrespectful attitude.
Also, our feedback for the service enhances the enthusiasm from the experience in the place and =  place, help a restaurant become better.