I've been told by trailer dealers and tire dealers as well to put your best tires on the right (curb) side of the trailer because that side runs along the edge of the road and takes more abuse from potholes and debris, especially if you run on alot of bad roads. I do agree with Doc, though, if your tires are wearing faster on one axle than another, you probably need to look closely for axle or brake problems, but yeah replace the worn ones. One other thing to look at is if your trailer is not pulling level - often older trailers will set high in the front because we're pulling them with taller trucks than they were designed for. If that's the case, and you can't adjust the stem at all, or not enough to level it out, then put the best tires on the back, 'cause it's carrying the most weight.