hi, i've tinted the rear windscreens on a few cars now and have never had any problems caused by the rear demister. although i bought some spray tint before, and when using it on an area that size i found it very hard to get it on evenly, so i ended up cleaning it off and using a roll tint instead.
personally i found the only useful applications for the spray tint are small rear quarters or lights, and i use roll tint on anything bigger