My auto machinist of many decades has been telling me that many of the new parts he gets in are cr@p. All of us need to be aware of this. If you let your shop buy the parts you will pay a bit more but the shop assumes responsibility for the parts and the installation,
AND he can make a little profit on the parts to cover all the extra work. Auto machine shops are not a good business to be in anymore, and you can look for them to gradually disappear, so if you have found a good one, treat them well, learn and show them that you understand their problems, and they well show their appreciation.
If on the other hand you buy your parts from one of the big cut-rate mail outfits like Jegs or Summit, YOU are responsible for dealing with parts problems. So if you are going to supply parts to the shop, you better get the full set of precision measuring equipment and get very good at using it. If you are thinking, "Oh I know how to mike piston skirts," you don't really get it; that's just barely a start. Incidently, unless you're satisfied with buying crate-motors or getting the shop to build your engine, you need to develop your precision measuring skills to double-check the work that comes back from the shop, because even good ones make occasional mistakes. I once got a Ford Fiesta block back from being bored at a good shop (they also did Top Fuel engines), and found that all the new bores had a huge amount of taper. Turns out that a recently-hired CK-10 operator (who should have known better and should have checked better) had goofed in his set-up. He was supposedly knowledgable, and he was very embarrassed and fixed the problem (with a 2nd over-bore), but he didn't last long at that shop.
Anyway, read this article, by the late great Joe Mondello, one of the best and busiest builders of racing and hot rod engines:
http://mondello.com/page24.html