I deploy one in the rear. Why? I like my 6x9s on the rear deck. I don't sit in the back seat, so I don't know if it impacts heat buildup.
I do think it helped scorch a vinyl cling stuck to the inner glass back there. I'll have to scrape it off now.
Come to think of it I have one for the front too. Forgot I had it.
Nope. I'm sure it helps, but not enough to really matter, especially for the hassle of dealing with it. I have the small driver side rear window "blacked" out with a piece of mylar. I doubt it does much, but it doesn't hurt anything there either. I may cover some other portions of glass I don't use, but there isn't much left.