I'd say taboo. What most people don't think about is how much it costs to be in business these days. For example a small specialized shop,with 5 or less employees may need to generate about 20 grand a month in sales just to be able to open the doors. A shop can't make it on labor alone,profit on parts helps keep things going. You don't take a 1/4 pound of meat and two slices of bread to the local burger joint and expect them to cook it for you do you?
At our shop,we do sometimes install a customers part if it's something unique,like a stereo,or performance parts,for example, I have a 16valve 190 in the shop right now that the guy bought a "remus" sport muffler for it. I do not sell these types of mufflers but my customer wanted one so he bought one and I will gladly install it for him. Everyone is glad to know that there is a "specialist" in the area when there is a strange problem with their car, they just have to remember that in order for the specialist to be there for you when you need him for something tricky, he needs your support for the little jobs too.