There was a HUUGE thread on this like last year that got closed. If you receive a sealed box and such, chances are you are 100% good to go.
Now if you get a code emailed, it WILL register fine. Problem is that maybe 4-8 months down the road, someone purchases a sealed AC from Apple (Best Buy, etc) and registers the SAME code, it will FREEZE both applecare accounts, and they will ask for proof of purchase, which would include the receipt, most likely a box and/or the TWO PIECES of registration papers inside the AC box that have serial/registration numbers on them.
I got my AC from an apple store for $239 for my 15" MBP. Student discount price is $239, I think retail is like $389 or something?