All emergency vehicles should have full 360-degree coverage. This is mandated by law many places. The simplest way to achieve this is by using a beacon or full size lightbar. However, a minibar will work just as well in most cases.
If you drive a tall vehicle, remember that the beacon can be hidden to vehicles close to you, and you may need two beacons diagonally mounted.The same problem applies to minibars and lightbars, so you might need to add some additional lighting.
If you replace the roof-mounted light with interior lights, remember that you still need adequate warning lights facing to the sides.
Also: consider the "viewing-angle" of the light you install: a vertically mounted TIR-style module will give a very narrow "field of view", where as a horizontally mounted linear-style module wil give a much wider "field of view".
When navigating through dense traffic, remember that only a small part of your vehicle may be visible to oncoming drivers.
Since anyone can flash their headlights, it is recommended to also put warning lights towards the corners of your vehicle. The same problem applies from the rear when stationary in traffic. This can be achieved with HAW (hide-aways) in vehicle lights or surface mounted, lights under or on the sideview mirrors, a wide lightbar or decklights in the corners of your windshield. ...to mention some.
Front Intersection Warning
One of the more dangerous situations when responding is navigating intersections, especially red lights or intersections where others have the right of way. If your vehicle is partially hidden by other vehicles, your lightbar or sidemounted warninglights may not be visible, and your siren is pointing forward. Hence, it is recommended to mount some form of outward-facing warning lights towards the front of the vehicle.
This can be achieved with HAW (hide-aways) in vehicle cornerlights or surface mounted on the side of the bumper, outward-facing modules on the license plate or in the grille or on the side of a pushbumper for example.
Avoid flashing or interfering with your turn signals as you ought to use them in communicating with oncoming traffic when driving, even on response (standard curriculum in emergency driving courses)!
Some lights will cover both width and front intersection warning, such as properly places HAWs in vehicle cornerlights or surface-mounted on side of bumper. A very minimal install could look like this: Nighttime low-power option If your vehicle is equipped with a lot of warning lights, it is highly recommended that some or all of these can be turned down during the darker hours, to avoid blinding other drivers.
The idea that blinded drivers will slow down is false!
Some simply carry on, either from stupidity or confusion, and will not be able to see officers in the best of reflective gear. Studies have also shown that drivers tend to skew towards where they are focused. Think of your car: it has driving lights and parking lights, and in driver-education you probably learned to turn off your driving-lights when parked, to allow for other drivers to see the contours of your vehicle and any people or other vehicles next to or behind it. The same applies to warning-lights. DO NOT BLIND OTHER DRIVERS!
Final Pro Tips:
Finally, always check with local regulations and abide by them! If local regulations require your vehicle to meet a certain standard; comply with it!