That depends on how you look at it. Naturally I would say its not a bad advice, being yourself is the best you can do for yourself. It saves you the stress of trying so hard to be something you're not; and people would appreciate you better for your sincerity. It the contest of business I'll say its bad advice. You have to create a picture for your clients to see. The image they see isn't what's real its the impression you give. Someone once told me

you have to fake it to make it.

If I'm pursuing a big contract I do not be myself I become the person they want me to be. I rent a better car, get a better suit, book a five star hotel for the meet and at the end i earn myself a life changing contract. So "be yourself" is both good and bad advice