There are no absolutes. Even sidewalk-riding (no!) which I generally oppose, has a definite role in parts of urban LA where it is by far the safest place to be for shorts stretches. (Sidewalk bicycling is actually legal in LA - although I don't recommend sticking to them unless you have to.) Even helmets (gasp!) might be optional, if you look at how few commuters wear helmets in heavily cycled parts of Europe where bikes are the main mode of transport in dense urban areas. (I still recommend 100% helmet use!)

Be cautious, be visible, and be predictable. You can figure out the rest according to your needs with experience and your route.

PS - Also agree with the "no confrontation" approach with cars. I'm sure there will be the rare d-bag who absolutely needs a verbal beatdown in public, but in general, you're not going to solve anything, and you're greatly increasing your chance of personal injury. A simple firmly stated request or even a disappointing head shake will suffice in most cases. I've been behind 2 commuters who got "buzzed" in rush hour traffic (likely unintentionally) who then rode up to the vehicle at a red light and physically harassed the woman inside, even banging on her window. That's totally unacceptable, childish behavior, and I was embarassed to be seen in the same 2-wheel community as them.