Depends on how much you play, I suppose. Now that my daughter's born, I have a lot more time to think up decks, and a lot fewer opportunities to play them. So I went ahead and used my "secret deck" for the Iron Chef tournament of champions this week, and it will break the format, even though I will only have played it at league or, if I'm lucky, one or two city championships.
And I'm not particularly secretive about it. It's name is in my details to the left of this post, and the list will be posted in its entirety once Patriarch judges the decks. Heck, I'd reveal the whole darn thing now, except it is that list itself that I'm competing with, and so I can't collect the glory for it quite yet. But I will...oh, yes, I will...
To answer the question at hand, I think you're best off not keeping your deck "secret" at all. There is no substitute for live-action playtesting against a variety of opponents, and even doing so does not necessarily mean that they will either tech against you or copy you (which, I think, are the two biggest fears secret-deck possessors have). Far better, I think, to have a deck well-tested and thus made consistent than to whip out something unproven for a major tournament, especially in the current metagame, when there isn't really a metagame. The deck you're going to jump out and surprise with your dominating secret deck might not even see play at the tournament you're in. Got a deck that kills Bandoom? What if you never face bandoom, and you end up getting shelled by some random Meganium d?
Of course, if you do have a secret deck that you think is format-breaking, the first thing you should do is make a semi-cryptic pretentious post about it so that you can get those all-important dibs on the intellectual property rights to it, and be hailed as one of the greatest deckmakers of all time. That's really, really important.
