The reality of it is, 90% of mechanics in any field try to upsell things and do extra work that is not necessarily needed, to make more cash. Although being a service mechanic I do know that sometimes it is better to replace a small part here and there when it is beginning to wear(not necessarily broken). The worst thing you can have is go in for a yearly maintenance on a unit and it breaks down 2 weeks later because you didnt want to take the time to replace a 25$ part. The thing about this, I always give the customer the option. For example, Ill say I have a safety device inside the unit, that is building too much resistance and may fail in the near future. Your unit is working perfectly right now but I can not guarantee that this part will hold up another year. At least this way, your covering your ass.
As for pissing in somebody's house, that is just disgusting. A guy in our company was fired for leaving a few ceiling tiles out and a few footprints on the ladys floor.
With my company, if you take too long on a service call they will take the hit and not the customer. My boss countless times has reduced work orders from 3-4 hours down to 1.5 or 2. We get bitched at for taking too long obviously. But his honesty with people really has built us a reputation. This year we have 6 jobs totalling over 5 million so far. All from word of mouth.
There are shady mechanics out there in every trade, just got to be able to see through peoples bullshit. I have gotten very good at it. Ask detailed questions, get them to explain everything, and listen to what they tell you. If it doesn't make sense in your head, chances are it doesnt make sense at all. Get a second opinion before having any work done if you are ever unsure.