My choice of bike shops is determined by whether the staff knows what they're talking about, or at least care enough to find out.
I went into a store recently looking for a new rear derailleur. I walked in about 15 minutes before closing time, and the manager rushed over to lock the door behind me. That set the tone immediately - "We're about to close, so don't take too long." The sales clerk told me that Shimano doesn't make a long-cage 105 derailleur. After his manager corrected him, I asked him how long it would take to come in if I ordered it. "I dunno, maybe a week." Wrong answer, and wrong shop for me.
I went up the street to another shop. I walked in less than two minutes before closing time; the clerk pulled a long-cage 105 out of the case, answered my questions about it, and rung it up five minutes after closing. He never mentioned that they were supposed to be closed - I was a customer, I was there, and that seemed good enough for him. That's the shop I'm going to go back to again and again. The part was $10 more at the second shop, and I'm more than happy to pay the difference to get good service.
The funny thing is, the first shop has 27" tires, and the second doesn't.