This is definitely a case by case. My shop is smaller with 2 Full time techs, 1 SA and myself. Customer are always talking to my techs simply because they are friendly and will say Hi to our customers. We get to know our customers and ask how their weekend went or even that vacation they just went on. I pay my guys well and I do lots of "extra" things for them. They are my best asset and I put them first before anything (except for the recent argument of A/C in the shop ) My guys know when someone is sniffing for something and will be very vague with someone like that. Usually don't see that person again. My regular customers don't ask, If they don't think they can do it. They bring it to us.
Now when I worked at the dealership, I had this happen a few times. I simply stated, if they didn't know how to work on the vehicle that we had trained technicians and we could schedule them in. If everyone hasn't read, I received an article sometime this year stating 2012 was the highest amount of deaths/accidents due to DIY car repair.