Lights on, always
RCW 46.37.522: "Every motorcycle and motor-driven cycle shall have its head lamps and tail lamps lighted whenever such vehicle is in motion upon a highway."
Whenever in motion. Not at night, not in poor visibility, always. Most modern motorcycles wire the headlight to the ignition precisely so this is not a decision, but if you ride something older with a headlight switch, the switch is not really optional on a public road.
Headlight height and beams
RCW 46.37.523 requires the head lamp to sit no more than fifty-four inches and no less than twenty-four inches high, measured from the ground.
Every motorcycle other than a motor-driven cycle also needs a multiple-beam system: an upper beam revealing a person at three hundred feet, and a lower beam revealing a person at one hundred fifty feet.
The twenty-four inch floor is the one that catches heavily lowered customs and some minibike-style machines.
Tail lamp, plate light, reflector, stop lamp
RCW 46.37.525 requires at least one tail lamp mounted between fifteen and seventy-two inches high, at least one red rear reflector, and at least one stop lamp.
It also requires a white light illuminating the rear license plate so that the plate is legible from a distance of fifty feet to the rear.
Fender eliminators and tucked plates
This is where a lot of otherwise tidy builds go wrong. A fender eliminator kit typically relocates the plate and, very often, deletes or obscures the plate light. If your plate is not lit and legible from fifty feet, the bike does not comply, no matter how good it looks.
Separately, the DOL states that plates must be mounted horizontally and remain visible. A plate angled steeply up or flipped vertical to look cleaner is asking for a stop, and it is a trivially easy thing for an officer to spot from behind.
If you want the look, buy the version of the kit that retains a plate light and mount the plate flat. It costs a little more and it removes the entire problem.
Why conspicuity is worth more than compliance
The reason the always-on rule exists is that the single most common car-versus-motorcycle crash involves a driver who looked, did not register a motorcycle, and turned across its path. A lit headlight in daylight is one of the cheapest interventions available against that.
What compounds it is position. A headlight in the middle of a lane behind another vehicle is hidden. The same headlight in a lane position where it appears in the driver's mirror or windscreen is doing work. That is a trained habit, not an equipment purchase.