When flying at night, do pilots keep cockpit lights on to help read knobs and switches, or lights off to keep night vision?

Just to give you a brief overview the Cockpit lights in most of the modern airliners typically constitute the following elements.

1. Overhead Lights
2. Integral Lights
3. Map Reading Lights
4. Instrument Panel Lights

Though the positioning and the design of the lights vary with the Aircraft manufacturer and the variants, the overall aim of the design remains fairly constant,

Overhead light

Typically they illuminate the entire cockpit to a comfortable level of ambient lighting. As the difference in the luminousity outside and inside is quite stark in this case, identification of external features and threats is a real problem. Hence the overhead lights are used typically in Cruise, when the perception of external threats such as weather or traffic around is at a minimum. They are generally turned off when landing at night, or scouting for runways during low visibility conditions

Integral lights

These lights flange the switches and knobs and illuminate the labels that are associated with the knobs. They ensure that the switches remain fairly well lit when the ambient lighting condition in the cockpit is low.

Map Reading lights

Designed to illuminate the chart holders. As pilots we rely on a set of charts to understand the airport layout and the communication frequencies at every airport. These are typically placed either by the sides of either pilot (as in the case of the Airbus) or at the control column (in the case of the Boeings). A light focused on the charts is essential for the pilot to refer to these charts at night.

Instrument Panel Light

The Instrument Panels in the traditional aircrafts (which still rely on needles and dials) or the Standby Instruments in the case of the modern airliners (which rely on LCD’s to display the various parameters and rely on dials and needle type of display as backup) are lit up with an Integral Light to enhance visibility.

Typically the intensity of these lights are individually controllable by specific potentiometers in the cockpit.

Though the lighting levels vary depending on the flight crew’s comfort level, generally most them prefer to have as little a difference between the cockpit lighting and the ambient light outside so as that there is no lag in changing from internal reference to external visual references. So the general practice is to keep the overhead lights to the bare minimum or preferably to turn it off especially during take off and landing.

The Integral lights are turned up to a comfortable level so as to assist the crew in locating the switches and knobs and just about dull enough so as to ensure that the transition from the internal lighting to the ambient outside light is not difficult.

The map reading light though is focused to be non-intrusive often bounces of surfaces in the cockpit and can be annoying at times. So they too are turned down to the lowest intensity.

Leaving all the lights aside, as pilots we rarely depend on lighting to identify switches. The sense of muscle memory works incredibly well in most cases, helping us extend the arms to the desired length to reach a specific switch or knob in the cockpit.

So to sum it up, the perfect lighting is a trade off between maintaining visibility inside the cockpit and reducing the effects of transition between the inside and the outside lighting conditions to a bare minimum.

Edit 1:
This is how I prefer to have the cockpit lights at night. Turned down to the bare minimum. Just enough to see the instruments and panels.