If you want wider tires on the rear for performance reasons, save your money. The general consensus is that it won't be of enough help on an LS to offset the costs because of the LS's suspension and near-perfect front to rear weight distribution.
If you want them for aesthetic reasons, go for it. There are limitations on the rear for clearance reasons, but I don't recall what they are (I'm pretty sure that 9.5" wheels are one step too wide for the rear, at least in the 19" size). 245s seem to have no clearance issues, but the 255s seem to be a mixed bag--it seems to be a function of the individual tire models; reports are that some 255s fit well, and some have clearance issues.
Also beware that running larger tires on the rear could be problematic, because you may wind up with different radii on the fronts and rears, which can cause problems with the traction control and/or ABS systems.