Does this mean you're cheating? Not if no one finds out about it. After all, doing any sort of card trick is "cheating" in a sense, using either clever ruses, sleight of hand, or speed to deceive your spectator into thinking you just performed an astonishing, "impossible" card trick.
A marked deck is useful because it helps you identify which cards are which without having to flip the card over. Watch this tutorial to learn how to mark a deck and the system for identifying which mark aligns with which type of card.