Many on here just don't care if there's a difference, and they haven't had any problems yet, but time will tell. FWIW, Goodyear has a maximum tread difference policy for AWD vehicles, and will not mount a single new tire if it is too far out of match.
When I lost one tire and has less than 15k miles on the rest (and had rotated religiously), I got a replacement from Tire Rack and had them shave the new tire down to match, for $25 extra. Take a tread depth gauge and measure every tire. If you have even tire wear you can tell Tire Rack the depth and they'll know the new tire depth and will reduce it to match.
If you don't have even tread wear, don't even bother.