You can fold your hand anytime you want when the action gets to you, if a player has made a bet and you don't want to call then you don't have to. Even if the hand has been checked to you, you can still check out of the hand if you want to, although i would not advise that you ever do so. Even with good hands you sometimes have to fold your hand, the only time i wouldn't lay down my hand even though if i thought i was beat is if i was pot committed, this means i have so much money in the pot that even if i think I'm beat i still have to call, this is called a protected pot.