It really just depends on the brand, model, size. Tirerack is great because they have a huge selection, dozens of brands, local dealers tend to have a few specific brands so cross shopping becomes driving/calling different shops, and then you get different total costs with the services/fees. It also let's you buy tires independent of the service side - there are some great shops in terms of service, but don't have the buying power of major chains for the tires themselves.
Also, there are some great alternatives to TireRack, who might have better shipping costs (some are free), no tax (depending on your location), like DiscountTireDirect, 1010, and if you're buying "specialty" type tires, there are online sources for specific vehicles that also sell tires (lots of offroad/T4R online stores with tires). In fact, my recent tire purchase was from a big online store that caters to late model domestic performance cars, so they sell tuners, headers, etc., but also wheels and tires - I had free shipping, a 12% discount, they beat anything local and other online sources by a good $300 and I got the exact brand/model/size I wanted.