A note to those with trucks
Doing this makes it nearly impossible to see where your wheels or bumper is. Should you be hauling enough junk to block completely your rear-view mirror, be sure to adjust your side-view mirrors accordingly. Mirrors set this way can also make parallel parking rather difficult, depending on your vehicle and how well you can already parallel park. It should also be noted that even when you set your mirrors up this way, do not rely on it solely. It is possible for a very small car or a motorcycle to slide into the small blind spots that remain. This can be a very bad thing.
Footnote: Here I thought I was a genius when I figured this out one day...and I thought it'd be a great thing to node. And here I find Mshadow and RevJim23 beat me to it.