I'm admittedly a cheapskate and a tinkerer, and I would let those tires keep rolling until the first threads show through. I did just recently splurge on the best possible tires (for comfortable, flat-resistant commuting): Schwalbe Marathon Supremes, but they are quite spendy. Before that, I got more miles than I expected from used tires that I bought for $5 each from a vintage bike shop (essentially a co-op).