Trust... but verify.
I don't believe it's possible for any mechanic to become so expert in his trade that he can handle all situations. Much too sophisticated. The dealership usually has multiple mechanics, and the chances of getting the proper diagnosis on questionable problems is much better. The other part that's good about dealerships, is that most (not all) use flat rate charts for services. That means more expensive, but maybe not as expensive as the local guy's hourly rate when he tackles repair of an airconditioner diverter... and is doing it by "feel".
I hate all auto service... just because until I got old, I did all of my repairs including engine rebuilds. I particularly hate the dealer service manager, when he says to me... "As soon as your car crosses that white line, the minimum charge is $95."
We all have horror stories... five years ago, my Town Car had a steering squeak that I couldn't find. Brought the car in, and received a call at home an hour later from the "Service Manager"... Four wheel brakes, rotors, shocks, ball joints, suspension compressor, power steering box trailing arms, etc, etc... for a total of $5500 plus tax.... and, in order to get my car back without having all the work done, would have to sign a legal release absolving the dealership of responsibility if I got into an accident.
Smoke pouring from ears and nose, went and got car... svc. mgr. hid in office.
Buddy and I jacked the car and used a hypodermic needle to lube a pitman arm... that was 45,000 miles ago... haven't changed a thing, no tire wear, no squeak. Saying that, though... of the 500+ members of my senior community, there would only be three or four who would question that diagnosis.
But if the A/C heat flap fails... It's the dealer for me... means stripping to the firewall, front and backside to replace a $.15 part. 10 hour job for an expert.
Current challenge is leaking air suspension.... aaargh!