I show a lot more interest in cars these days than I did when in my 20's when I used to trust the garages/mechanics. I now check things out myself and keep myself informed as to what services are due, when, and what else needs to be checked/replaced.
We "should" be able to trust garages to do the job right, but at the end of the day, it's the car owner who loses out when, say, a garage puts in the wrong oil or doesn't change the timing/wet belt to schedule as they'll always find a way to wriggle out of any responsibility.
Also, using a dealership garage is no guarantee of them getting it right. Some "back street" garages are just as good, if not better, as they'll often be more experienced and are more likely to be "proper" experienced mechanics than just young "fitters" used by main dealerships.
Quite impressed with our village "grease monkey" garage around 15 years ago when I first booked my car in for a service with them, and the receptionist at the desk was very thorough, checking the manufacturer's service schedule to list down exactly what needed doing at that age/mileage, checking the handbook for what had already been done, and then flagging up the timing belt was due the year after, but that it would be better to do a year sooner to coincide with a full oil/oil filter change as it would have to be done again the year after if we had the timing belt done the year after. She also flagged up that a different kind of engine oil was needed for that make/model, rather than the usual one they use for most cars! I think that was the point in time when I realised there's more to it than taking it once a year to a random garage for "a service" and that there has to be a bit of owner responsibility to keep yourself informed. Very impressed that a receptionist went into so much detail rather than the usual "is it a big or small service?"