Unless you are doing some heavy duty boondocking, there is little reason to have more aggressive tread on the rear. If you are talking about the same tread design but some tires having more tread than others, I would put the ones with more tread on the front to improve wet pavement and braking control. However, as mentioned, normally the amount of remaining tread will not play a big part. With normal wear, the tread will not be too worn before the tire age dictates replacement. Probably the biggest consideration as far as tread is concerned is matching tread depth on all four rear tires. If there is much difference between the two tires on a given side, the larger diameter tire will bear greater load. If the difference is from one side to the other, excessive rear end wear can result.