I always rotate, but I try to do it before the rear starts to flatten out. Usually 500-1000 miles, depending on how fast the tire wears. Then I replace both once the rotated rear is done. I wouldn't want to put one new tire on the front and keep an old one on the back. I'd rather just replace both once the rear is done, even if the front looks like it still has life left. Don't want to turn this into a tire debate, but I can get a ton of miles out of 4000s II's, so I'm not concerned about getting every last bit of tread out of every tire.