I know the Police up in Scotland will pull you over for it.
I had a pal who was a joiner and had his stuff on the windows at Rear window and he got pulled over as it obstructs his view and as the car was designed with a rear window it would not be safe to reverse and etc, Me personally thinks it rubbish as I drive a van with no rear windows and it can be hard to reverse with blind spot.
Best bet is to speak to a sign writer and they often know. Plus have a look at maybe getting the car wrapped instead of the cheaper versions of letters. I got Van Wrapped and it is just soooo Much better!