I'm friends with my old mechanic, and the stories he tells about people coming in and explaining to him what his job is because they read it on the interweb is actually pretty awesome. He'll have rednecks marching in demanding that they fix problem X because that's what the internet said was wrong, then asks them if he wants them to fix problem X (which isn't broken) or the actual problem.
There are plenty of ignorant and negligent service people out there, and they'll usually appreciate a heads up on what they're looking for, but I usually try to give them the benefit of the doubt before telling them how to do their job.
Current: '16 VW GTI
Previous: '15 M235i, '13 335i, '08 TL-S, '00 Corvette