I follow RUSA rules:

Article 10

For night riding, vehicles must be equipped with front and rear lights attached firmly to the vehicle. Lights must be turned on at all times during hours of darkness or other low-light conditions (rain, fog, etc.). At least one of the rear lights must be in a steady (rather than flashing) mode.

Since I use an LED headlight on a dynohub, I just never turn it off, that way I'm always in compliance. My current rear lights are twin Superflashes, one steady, one blinking. I'm going to get a wired tail light soon. I'm deciding between the B&M 4D Lite Plus and the Supernova (which unfortunately is only a rack mount)