Well, I suppose you could say I'm a mechanic, considering that I do work on other people's cars for money, but its not my main profession, and I certainly don't do it regularly.
So as someone who works on other people's cars, here's something to consider. I've offered to do plenty of simple jobs for people, and alot of times I *DO* find lots of other things wrong that do need to be fixed right away. This is typical thing people fear when going into repair shops and most people feel like they're being ripped off just to make the bill bigger. I've almost always found other things wrong when doing jobs for customers that I have to go back and order more parts for before I can return the car, and I'm not that experienced. If it happens to you, don't automatically assume that they're making stuff up.
That said, yes, I usually fear taking the car in for repairs, but not because I'm afraid of what they will do. I'm afraid of what they WONT do. What I mean is, I'm actually very experienced working on cars to the point where I've removed a few cylinder heads in my time. If I'm taking my car in for something, its going to be for something small and troublesome that I don't have time to troubleshoot (like eletrical gremlins or vacuum leaks). When they get lazy and I get cop-out answers is when I get mad. For example: I had a tire wearing problem in the rear that I knew was caused by my sagging suspension and I was riding on negative camber. The whole reason I came to them is because I didn't have the bushing press to replace the bushings I needed. Their answer was that I needed an alignment, even though the rear of my car doesn't have alignment adjustments. They didn't take the time to properly identify the problem, and that makes me mad.
Bottom line is that it works both ways. The best thing you can do is be informed about your car and know a little bit about the business.
Here are some tips:
AUTOZONE READS CHECK ENGINE CODES FOR FREE!!! If you get a check engine light, and you're not under warranty, use this feature! It saves you the diagnostic fee when you go into the shop, plus you give them an idea of what to look for (and what parts to have on hand).
Car shops do typically over-estimate your needs for brakes, at least in my experience. Better to err on the safe side I guess.
Electrical and computer systems are typically the most aggravating and at the same time least profitable jobs. Don't be suprised if they don't get the problem right on the first try. I'm a mechanical engineer that got an A in circuits and I still get it wrong sometimes.
Warranty work typically pays less (for the shop) than customer out of pocket work. That doesn't give them the right to do a crappy job, but realize that they're going to try to spend less time on the job than if you were to pay, and the more experienced techs (on the upper half of the totem pole) don't have to do those lowly jobs. Not much you can do about this, just something to be aware of.
I know its nice to keep your seats and mirror settings where they are, but since the tech gets paid by the job and not the hour, I'd much rather have him mess up my mirrors and make it easier on him so he can spend more time on the job than for him to be careful.
Just some food for thought.
Cliffs: Its a hard job, I've done it, but watch out for the "one size fits all" response that really doesn't address your problem.