On a gas engine I wouldn't even think about covering the front up, too much excess heat is already being produced. On a diesel it's mainly to help the engine get to oper. temp. faster and help them stay up at that temp. I do this on on my 92'. Took a piece of cardboard and cut it the size of the radiator, cut about a 10" hole in the center to allow cool air to flow to the fan clutch so it doesn't burn up, cover the cardboard with duct tape to somewhat weatherize it, and then slide it down in front of the radiator. It works great, gets the truck warmer faster and helps it hold the heat it's built up a lot better. On my 06' I have the factory Mopar cover that goes under the hood and covers the entire front end...radiator, intercooler, all that stuff. It's made of a vinyl like material and has a large probably 10" x 16" opening that has 4 triangular flaps that can be closed or open to control how much air flow gets to the engine. I usually run with about 2 closed in anything below 45 degrees when I'm unloaded. Loaded I take it off because the truck has no problem maintain heat then, plus the cover covers up the trans cooler and that wouldn't be a good thing to do when towing.
2006 Dodge 3500 w/ 5.9 Cummins
Scag Turf Tiger 60" w/ 29 hp Kaw.