there's a guy at my school with a 79 hatch that used to have a big running pony in the back window...he took it off the second day of ownership though so its all good.
Cheeeeesssyyyyy. I think one, MAYBE two, small, well placed, subtle stickers are okay. My buddy has one in his back window that says "Boost Gets You Laid", I like that one. The other one I like I think Aliate X has, the eat, sleep, race one. Where did you get that btw?
I've had mine on for years. Back when there was a "mustang gang" in my hometown, pretty much everyone had the pony. Opinions are like buttholes, everyone's got one. Do what you like, it's your car.