One TB owner put Rain-X on his windows and they went dark.
Just one data point, but I would not use rain-x on any plastic window. It's pretty effective on glass though...
I use Plexus. It works well, gets the bugs off, and polishes out quickly and well.
The most important thing is to use a very clean cloth. If it has been dropped, it must go in the bin, or be used for wiping oil off things. It's very easy to scratch windows.