It makes sense, considering that the first order of business is to keep blood from running all over your one good T-shirt. Sure, bloodstains look badass when they're new, but when they dry, they just look like you dribbled brown gravy all over yourself.
But Actually ...
Not only does holding your head back not do anything to actually stop the nosebleed, but it grants the blood free access to flow down your throat, which is actually a worse place for it to be than on your Dragon Ball Z shirt. Sure, you might figure that blood is supposed to be inside you anyway, so it can't do any more harm down there, but actually, this technique can result in choking, or, if the blood travels further down, stomach irritation and vomiting. All things considered, you'll only wind up grossing everyone out even more.
"... and then I barfed up a gallon of my own blood. She said she'll call me."
What Can You Do Instead?
The consensus among doctors is that you should sit down, pinch your nose, and lean forward. Leaning forward will help you avoid swallowing your blood, pinching your nose will prevent the blood from escaping it, and sitting down will help you not be standing around like a jackass while waiting for the bleeding to stop. Most nosebleeds will stop by themselves within 10 minutes, so you can usually ride them out in this manner without making your house look like the finale of Django Unchained.