Originally Posted by
alathIN
I've seen this advice a lot of places, including if I am not mistaken from Sheldon Brown.
The idea is that the front tire does more of the work of braking and steering - safety critical functions - so you don't want to move a worn tire from the back to the front.
It is not so much how much wear there is on the tire. I am more worried about a catastrophic tire failure on the front than on the back, as on the back I have a better chance of avoiding a crash. I have had tires go bad with sidewall failures and a few times with casing tears under the tread, most often on the rear. And for that reason, I really do not want to move a tire that was on the back to up front.