Joining the popular answer...Never.
I have only replaced tires in pairs to date. I guess if I had to replace just one and the other was in really good condition I would follow advice and put the good one up front, however most likely I would put on a new pair and keep the old "good" one as a spare, so if I had another tire need to be replaced before its pair I would just swap one out till I was ready to get a new pair.
At the end of the day, I don't find the cost of bike tires to be much compared to the cost of my alternate commuting options.