Always go with a Klaw for your Nobz in either biker or normal boyz squads. You need this because Orks are not terribly good in close combat against certain enemies. Walkers, heavy tanks, and units with 3+ or 2+ saves can usually fend off a charge from Orks. You need the Klaw to be able to break even in these situations.
Without that Klaw, your Orks could get tied up in combat with a Space Marine Dreadnought you can't hurt. You'll also have a hard time against infantry hiding in transports. It's hard to destroy even an armor 10 vehicle in hand-to-hand with only ST 6(from the Choppa).
Another thing to consider: a charging unit of 10 Ork bikes will only kill about 2-3 Marines on average(30 attacks, 15 hit, 7.5 wound, 5 save). All it takes in this instance is for the Marine player to roll a little above average, and the Ork player to roll a little under average for the Marines to win the assault. The one or two outright kills you get from a Klaw will help make sure you are going to win the assault. It's even more important if you face a unit with a 3+ save and Feel No Pain.
Don't worry too much about the loss of Initiative from the Klaw. Even with boosted Initiative on the charge, most things in the game that can hurt you are already swinging first. You're opponent will also have to wipe out the whole unit to prevent that Klaw from getting its attacks.
For all Nob units, you don't have to give every Nob a Klaw. You can give most of the unit Choppas to save on points and get the most out of the wound allocation rules.