For me, I generally walk in with the part number I'm looking for already stored in my phone because I do look it up on the manufacturer's website prior to going to the store. However, I do allow them to do their job and will question/correct them if they pull the incorrect part.
Sometimes you will have people in there that will blow you away on their knowledge (in a good way). Other times, you'll know just to check the part number you're after and leave immediately.
I think it's best to walk in parts stores with an open ear and be prepared to hear things that are good and things that are not so good.
I remember a rep at a parts store telling me that the R134a retrofit kit they sold was all I needed to do a conversion. Never mind flushing the old stuff out. Just slap the kit on, charge it and I was good. That was advice I didn't take. Flushing and new o-rings is a must in my book. I just ask sometimes to see if there's something new that I don't already know.
On the flipside, I was repairing the smog pump/air injection system on my truck when I bought it and a few hoses were clearly disconnected and I bought two preformed rubber hoses from the parts store, but needed something to connect the two hoses and I was going to use a plastic fitting, but he strongly recommended going to a hardware store and buying a metal fitting. For all of my auto experience, he recommended something very simple that I never have to worry about ever again as long as I own my truck.
I have many other positive stories to tell, but my take on it is don't get so full of yourself to the point that listening to others is not an option because sometimes you'll miss a simple suggestion that may eliminate you from experiencing grief/problems in the future.