This is focused more towards POV's...Placement is very important. Some examples:
FRONT. The idea here is to get maximum warning light into the mirrors of the vehicle in front of you. That being said if your vehicle is say a car then adding warning to the low-to-the-ground fog lights may not be as effective as in a full size truck or SUV. Oncoming traffic should see you coming fairly easy if the drivers are paying attention. Also the human eye sees better vertically than horizontally so having different levels of light spaced out up/down (grille, dash, RVM or roof bar) can be more effective with less lighting if you are on a limited budget or restricted by law. Another thing to remember is that white light is brightest on the color spectrum and will wash out colored warning lights so keep that in mind when finding homes for grille lights.
SIDE/INTERSECTION. For POV's the best intersection warning I've found is clear strobes in the front corners of the headlight housings, seconded by License plate brackets. Draws attention to the rest of your warning package. As for the side (B post, truck tool box, etc.) its best to keep the warning close to eye level of other motorists, think between 3-5ft from the ground when possible. When doing an install I sit in a chair about 40-50ft back from the vehicle and move around it to get an idea of where the warning is going to hit other drivers at distance.
REAR. This is very important if you respond directly to scenes a lot and are parked on or near the roadway or are assisting with traffic control. I won't get into usage (turn off rear warning when using an arrowstick), just placement. If you use a separate amber traffic directional device, then it is best placed up high and away from other colors of warning so it can be seen by motorists further away. Thus giving them plenty of warning that something isn't right ahead and hopefully they slow down and move over before hitting your vehicle. When placing other warning lights the vertical thing as mentioned in the FRONT part above plays a small part here as well. Also keep in mind of the width of your vehicle, best to place lighting as far to each side of your vehicle as possible when facing rear (or in any aspect really) so motorists at night don't mistake your 1 ton pickup for a compact car.