IMHO It all depends on the condition of your front tire. It if is badly cupped I would change it. Normally I change them one at a time.
I usually get 9-12k out of the rear tire and 16-18k out of the front. Since I ride 95% two up(Heavy) I run 40lb front and 45lb rear to cut down on the cupping.
I've been through about 5 or 6 sets of tires. I change th tire that's worn out, th front & rear usually wear out at different times, on my Wing. So i change 'em one at ah time, at home on th Harbor Freight Tire changer.It's really jest whatever you wanna do, i don't like ta waste ah good tire if there's still safe milage left in it. :mrgreen: