I originally joined Costco because even after paying for the membership to join, I saved money on the tires I wanted over the local tire shops prices. Most places around here do the free rotation / patching, so it's not a differentiator. The Firestone place near me used to match TireRack's prices, but they stopped doing that.
Tires are something that I am willing to spend a little more on. In my younger days, I put the cheapest Cooper tire on my car when I had a sidewall puncture, and it was the noisiest thing ever. Whenever I rotated my tires, that one had to stay on the left rear to minimize the noise I would hear while driving. Now I will only buy Michelin. I think they give the best ride for the money.