In a situation like this, personally, I'd say that informing the nearest Judge would be the best idea. If you're not honest, in any game or sport, and people find out that you somehow cheated -- like the opponent making a comment about choosing the specific card that was different than the rest, and then winning -- then there won't be a bunch of people who would be very interested in playing with you (Especially if the issue persists), and getting a bad reputation, as well as not as many people liking you all too much... that's probably the harshest penalty that can come from the situation.
-- Basically, the harshest penalty is players not wanting to play with you, due to the one time you decided to take the curled card that stood out from the rest, which would be considered cheating in lots of areas.
Whereas, if you inform a Judge about it, the worst that will happen is either you losing the game, or needing to find a way to flatten out the card, so the difference isn't so noticeable. Also, since you were honest about something, then other players will probably see you as being an honest person, and would probably know that you wouldn't cheat in a game. So, in this situation, it's more about time/possibly buying some sturdier sleeves (I know that there are some, or were some, that were pretty thick and didn't curl/bend easy... don't remember what they were called)
-- Essentially, compare being honest, and picking up new sleeves/flattening the card/losing the game to being dishonest, where you lose the respect of those around you, assuming you're discovered.
But, in the end, it really depends on what is more important to you. Is winning the game, through any means, the most important thing, even if you're running the risk of being branded as a dishonest player/cheater if discovered? (-- Which isn't unlikely to happen, because I think that most people can put two and two together - there's a curled card, different from the rest, suddenly, you pick that specific card and win. Sure, there's no way for the opponent to "Prove" that you were cheating, but it can be easily assumed, and legitimate reasons can be brought up.) Or, in terms of what's most important, do you just like to play and have fun with the game? Sure, you may lose a few games here and there, but being an honest player will probably make the process of making friends, easier.
I also did like the school test example, used above.
~Nips