I agree with the best on rear theoretically…but practically speaking, I may put the newer ones on front if I couldn’t afford 4 and if the ones on back still had good tread and better than half their life. It’s more critical that the tires match in performance which includes tread wear, but also design, size etc. Other wise, if you don’t put the “slightly” better ones on front, you may NEVER be able to rotate the tires on a FWD car. So for me, it’s a matter of degree…big difference in wear, better on back; slight difference, I could do what’s expedient.