I have a Lund 4 piece stainless cold front. Haven't had a problem, and as it was said above, you get plenty of air through the bumper.
I usually remove mine when the days see 40 degrees for a high, or if I plan on towing hard.
I've seen people with cold fronts that procrastinate and leave them on even when it is 60 or 70 degrees out, and I think that's putting too much of a load on the cooling system.
The 4 piece cold front made a big difference for me last winter.