I'd do one of two things: Take it to another dealer for a second opinion, and or take it to a garage you trust and have them look at it too. If it is worn past the minimum thickness for the rotor, it needs replaced, pad too, because they proabably have an abnormal wear pattern as well.
I'm a little pessimistic when it comes to dealers... your comment about waiting till it's later in the warranty makes me wonder if they suspect that the rear calipers are hanging up and chewing the rotors early. They then can get by with 2 $15 dollar rotors, as opposed to rotors and calipers, and you're stuck with it the next time it goes belly up...
Unless you're a hard driver, ie speed and brakes... I don't know that I'd spend the money for a brake upgrade, particularly the rear, without doing something to the front first. Typically only 10% of the braking of a vehicle is done in the rear. The LS might be different because of better weight balance, but still it's not 50%. I dunno... that's just my two cents worth. Just get a second opinion that's the safest thing.
Kleetus
'00 LS V8 Sport