Nope dude. Master Key totaly break the game. With it you can just go before blightown and get sword and shield wich will drive you through half of game.
If someone plays first time DS they should not get master key.
The good thing about playing the first time, you have no idea where blighttown is, or what sword and shield you are talking about.
Now you might randomly wander upon things you shouldn't have access yet, but I'd put that more in the fun discoveries column rather than breaking the game.