If you read the article I referenced you would know that there are many variations of it. up up down down left right left right B A Select is the most widely accepted because it appeared in the most games. Start has never been part of the code itself but is often appended because usually the code is entered while the game is paused. Hitting start was necessary to continue playing.
Wikipedia also has a
list of games that featured the code. Note that its not always the same but they are all very similar.