I've got a good joke about the flex test and "used boards" but that's probably not appropriate here.
You don't need to worry about flex in my opinion until you notice it affecting the way your board rides. If you're a park dude and that board can't pop you anymore then retire it. If you love shredding the hill and popping out of turns and your board isn't springing then dump it, you only get so many winters.
There is absolutely no point in ever flexing a board by hand, that will void your warranty on any Burton products with infinite ride