Absolutely keep a replacement on hand. I keep replacement tires for all my bikes, 'cause you never know. (Of course I cheat, and buy my tires in sets of three, since the rear wears at about twice the rate of the front. So I have the replacement available right from day one.)

Meanwhile, it's now become a bit of a game to see how much I can get out of that tire. I'm checking it regularly for the first signs of the casing to show through.