I use both cold fronts. The bumper cover, which is harder to install, stays on all winter. It covers the two air scoops on my 2002 Chev., which directs air straight into engine compartment. Keep an eye for salt buildup under this one.
The grill cover, which installs much easier, I removed if temperatures go above 40 Deg F. Also, if I plan to tow a heavy load in winter, I'll watch tranny temp, and coolant temp closely and remove grill cover if necessary. The tranny cooler is completely covered with the grill cover installed. Performance may suffer with grill cover installed, due to the intercooler getting minimal air flow.
If I want a little more air flow when using the grill cover, I will only attach cover using the bottem clips, and the top snaps only. I can then fold under both ends of grill cover, which opens each end of grill a couple of inches, for a little air flow.
To make installation of either cover easier when cold, warm fronts in house a couple of hours, and they'll streich much easier when installing.