Well, I am pretty sure that on of the manuals (console/game/memory card) recommends that you be sure to completely quit your game and return to the title screen before being the previously talked abotu shudown procedure. This way, the system is not accessing the Memory Card in any way, and there is no risk of corruption.
Most games only access the memory card during specific points that are made obvious to the user with a pause in the game, even if only a short one, and some sort of prompt letting you know that the card is being accessed and not to touch anything. With these games, corrupted data shouldent be an issue, uneless you are neglegant and turn off your system when it ovbously tells you not to. There are some games that are periodically saving in the background without any sort of obvious stop in the gameplay. No games for the GameCube come to mind immeaditly, but the Halo titles do. They save every so often duing gameplay without any sort of warning promt or anything. So, a game like that, if there even are any on GameCube, would risk data corrution if you arbitrally turn the system off. Which, once again, this is why manuals recomend completely exiting the game to the title screen before shutting down.
ANd really, even if you do shut your system down "the wrong way" you should never really have a problem. Rarely do I take the time to completely exit a game before hitting the power button, and in the 5 years since release, not once have I had a problem. But then again, I only use Nintendo's offical cards...