I used to work for a dealer as a technician. You don't even want to know the half of how shady things can be. The worst part is, it is the fault of the warranty companies and dealer management that make it this way.
I was a dealer tech for about 5 years total. 99% of the dealerships out there are on a flat-rate system for their techs. What this means is that if a job pays 3 hours by the book, the tech gets 3 hours for the job, regardless of whether or not the job takes 1 hour or 12. Now, that being the case, in the 5 years I was at my dealership, I watched the dealer general manager drop the pay time on every single "by-the-book" job we had. When I started, brake jobs had a flat time of 1.5 hours, when I left, they paid .7 hours with an additional .3 if the rotors were machined. Timing belts paid 3.5 hours when I started, and by the time I left, they paid 2.7 hours.
These are small drops, but they really hurt your paycheck at the end of the pay period when they add up. Especially during slow times...
Now, couple that with reduced warranty pay times on jobs that are covered by warranty, and you have an extremely hostile working environment. The result is techs that are damn near forced to be "shady" to make a real living. Here is where the dividing line is though... Techs with a conscious will do their "scamming" on the warranty company itself, so the customer never has to pay a dime for it. Techs without such morals will do it to "customer pay" jobs as well.
The people that do it to customer pay jobs are the scum of the tech world and should have their right to fix cars or trucks in a professional environment revoked. It turned my stomach when I would watch techs make a recommendation on a customer pay ticket that would set the customer back for over $1k, and laugh about it when they never did the work because the car didn't need it in the first place.
I personally used to do "tack on" items. For example, if a car came in for something under warranty, I would do the job needed to fix the problem, but would "tack on" additional work items that were related to the problem. I wouldn't actually do these additional items because they weren't needed, but the customer wasn't actually paying for them. Yes, it was still wrong, and I never felt right about it, which is a big part of why I left the technician field behind me. However, it was pretty much a freaking requirement to even make an 80 hour paycheck every two weeks.
In other words, if your tech ever makes a recommendation and you are actually paying out of pocket for it, request a walk out to the shop to see the problem item first hand. Give it more than a quick once over too, because a smart dirty-tech will smear dirty oil, coolant, or whatever else is needed on the area to make it look bad, even if it isn't.