How far to back up when doing...what, exactly? The back window itself is pretty near where the bumper is, front-to-back. I don't see where having a marker on the hitch will help you find where the back end is much in general driving. For hitching up a trailer or whatever, that's a different story entirely.
The stick-on fresnel wide angle lenses work nicely at helping you see what's behind you during the day. They're not so useful at night, nor for judging the distance of relatively far-off objects. Given the price, though, I think having one is a no-brainer.