Nope, not true at all. There are many different styles of crankbaits that dive at different depths, though. If you find yourself fishing in ponds and smaller lakes, you probably don't want to be throwing deep diving crankbaits. They will likely dive straight down to the bottom and pickup all kinds of gunk and get hung up on you a lot. When you go to purchase crankbaits, pay close attention to the specs. Some will only dive down to a foot or so, while some are designed to dive down 20+ feet.
Don't add anything to a crankbait. Simply tie it on, and start fishing it. As far as knots go, keep it simple. The Clinch Knot and Palomar Knot will do just fine for you. I personally like using a Palomar Knot for lures like crankbaits; reason being, they're not quite as bulky as other knots, which means it won't mess with the swimming action of the lure.