If you can't react fast enough before they go past your crosshair then you aren't putting your crosshair in the right position. The only time you should ever have to actually flick is in firefights against multiple people, which you shouldn't be taking anyways; or if the enemy catches you off-guard, in which you got out-played and deserve to die or you were playing sloppy.
Crosshair placement will always get you more frags then flicks ever will. It always takes more time to move your mouse and then click than it does to just click. You lose that duel 90% of the time, so spend your time practicing something that will net you more frags, as well as more consistent frags.
Flicks are fickle, but crosshair placement can only fail you if you fail it.