If you had a consistent wind direction, then adding snow to the windward side would help retain heat. That said, you may wake up to find you are totally drifted in. It would take a lot of work to bury the car, so I'm not sure that just covering a bit of it would be worth the effort.
However, if you no longer were able to use the engine, then fully burying the car might be a great option, or bury three sides, and reflect fire heat into the fourth? If you kept a tire or two available for your signal fire, then you have a good option for visibility. Otherwise you have to be religious about keeping the top of your car clean to have any hope of being seen from the air(unless your car is white).
There are a lot of factors involved. But to be honest, moving enough snow to bury a car is going to take a massive amount of effort, and if you are trying to conserve calories, there are better, easier options. And from having seen pranks involving burying cars, it takes a surprising amount of snow to actually do that.
Also in my experience, areas with light fluffy and easy to shovel snow, tend to be warmer, and cooler areas have hard, icy wind-driven snow. (unless its really warm when it snowed, then you get the heavy, wet "heart-attack" snow) So in an area where you have greater need for the insulation, the snow is less likely to be ideal.
So if you had slid off a reasonably well traveled road after dark, and the snow was falling, and you knew that there was little danger of being hit (by another car or snowplow) then the best idea might be to let the car get buried overnight, helping keep you warm, then figuring things out in the daylight, instead of trying to dig yourself out in the dark, and moving all the of insulating snow away from the car, when its unlikely you are going to move without help. This means you probably can't use the engine overnight, (but you shouldn't sleep with the engine running anyway)
Just thinking about it, there are so many factors at play here. Its hard to know what would be the best thing to do. But there are a few things I have experienced.
Cars actually need very little air to run, but they do need some, and the best way to block the air is with a wet/snow-filled air-filter. If there is a chance that your air filter is wet, and you can get it out, then that will help. (be careful though, the airflow sensors are very important, and damage to them may keep the car from running at all, making this exercise pointless)
Blocking as much of the grill as possible helps as well, as you want to keep as much heat in the engine bay as you can. In fact, if you can hear the electric fan under the hood kick in, that means you are wasting heat through the radiator. The heater core of most cars is between the engine and the thermostat, so you want all the heat coming into the car, and none going into the radiator, if you can help it. Also be careful if there is any chance you damaged the radiator or hoses. in which case, running the engine is a waste of time, as the heat won't be transferred into the car. In a powerstroke F350, with an unblocked grill, and a -40 wind head on, the temp gauge won't even move at idle.
Most recommendations I've seen say run 15 minutes out of every hour. Lets say for the sake of argument that a full tank will let you idle for 24 hours. (the F250s and 350s I drove in the far north would do between 24 and 48 to a full tank) but that means that with the 15 on 45 off, 1/4 tank is only one day of fuel. However, fully warming the engine isn't helping you, so I would (assuming you have a good and trustworthy battery) run the car more often, for less time. Starting when the temp inside drops, and only running until the temp gauge moves on the dash. Running the heater fan without the engine running doesn't do much, since the water-pump isn't moving water, and it should only take a couple of minutes to top-charge the battery. My theory is that by keeping everything at a cooler average temp, but running more often, you avoid stressing the battery, as well as wasting heat.
Now if you have a Wabaso, or Espar heater, then its easy since those use hardly any fuel, and you'd only need to run the engine every few hours to charge the battery, and hang out in comfort.
Getting a bit long winded. Lots of things to think about, and since its 30*c out now, thinking about snowdrifts is a nice change...