Yes and no. Out of respect for my mechanic I let them buy the parts for my cars when I do repairs unless its a big ticket item. The only time that I would bring parts is in a case where I got an exceptionally good deal on a part and I would ask politely if it was ok while getting a quote on the job upfront. Many owners will call in and ask for a quote for parts and labor show up with the parts and expect the quote to remain valid. I have no issue with a mechanic that would be slightly annoyed with that as the profit that they make on the wholesale price they get on the parts in some cases helps them be more competitive on the labor rate as some jobs are more labor intensive than others but yet are "simple" repairs that most customers will balk at a high labor rate.

It depends on how it was approached. If it was a surprise when the car was being dropped off that borders on trying to take advantage of someone. If it was negotiated in the original price its another thing. For a first time visit to a mechanic I would never recomend bringing your own parts. You don't want a reputation of being a cheap skate. At that point your car becomes their bottom priority in the shop. You want your car to be their top priority and you wanted it treated with respect and care as if it was their own car.

I shy away from mechanics that come up with bizzarre reasons not to use customer parts but at the same time respect a shop owner/mechanic's need to make a living.