Yea, you should avoid picking the master key for your first playthrough .-.
That's just my opinion though~
Why is that your opinion? His opinion is to start as Thief and choose the Old Witch's Ring.
Because thief gives you the master key and the old witch ring is inconvenient to get so people won't usually ever discover what the girl says (even though it's extremely sad D:).
I feel that the game has different levels of difficulty, and you can do things to make the game easier and gain advantages.
One of the first advantages is the master key, but it also can cause issues. If new players picked it and just bypassed the depths to do blighttown rq and kill the boss, you miss out on embers, armor, and simply the experience.
If you are the type of player that doesn't mind OP advantages because you're going to do everything anyways then that's one thing, but to get the key just to skip one of the most hated areas in the game is bad.
Most players won't even explore blighttown with the master key, and they won't discover The Great Hollow and ultimately Ash Lake.
I just feel that it's a huge cheat. I'm not a fan of that stuff, but the one real advantage it gives you that I would agree to pick it as a gift is to fight Haval early on. No one likes to deal with being encumbered.
For me, I'd recommend all new players to avoid the master key and black knight weapons/shield. They really throw off the difficulty of the game making it too easy, and you end up creating bad habits because you had a strong weapon in your hand the whole time. After your first playthrough then it really doesn't matter, but I feel the first time you play you should play it in a certain way to experience what the game really has to offer.