What length should our dog's nails be clipped?

Our dog is a chow and he has dew claws. Someone told us that dew claws should be snipped off? Is this necessary? What are the best grooming tools for a chow? I am using a Furminator but he has some very coarse hair that is difficult to comb through on his back flanks and his tail! I've checked some sites but no help yet! Thanks!