No one ever commented about the rear window of my 90 LS not being tinted while the side windows had 35% tint. It looked fine and I enjoyed the clear rear view on dark winter days when I lowered the retractable shade. Attached is a photo of the 90 LS taken a few days before I sold it in 2003.
The non-metalized tint on the rear window of my 00 LS is a different shade of gray than the metalized tint on the side windows. I don't think anybody can tell and no one has ever commented. Can you tell? Look at the photo of my 00 LS showing both the rear and side windows from a distance - it's the photo labeled "with winter tires".
It is quite common to tint the front side windows to a different degree than the rear side and/or back windshield. Many states do not allow the same degree of tint on all windows -- here is a table showing the various state requirements: http://www.tintlaws.com/ And what about SUV's and minivans? They almost always have darker factory tinted windows aft of the front side windows.
As for a Lexus requiring a certain "look", the photo of the license plate frame on my 00 LS says what I think about that! If I "followed the herd", I wouldn't have bought my first Lexus LS 21+ years ago at a time when many people, including some of my friends, said it was stupid to pay so much for a gussied up Toyota.
I am sorry to bump such an old thread, but I have an update to tint laws listings - tintlaws.com hasn´t ben updated for quite a time and in 2015 it´s already quite inaccurate. Check out windowtintlaws.org for up-to-date listings.