This probably won't affect you since you live in CA, but everybody else who lives in a region that gets snow should heed this:
If you plan on putting on mesh grill covers that have a fine weave to them, take them off in the winter (or use a mesh with a wider weave)!
Had the scare of my life last winter here in New England w/my Eclipse. I had some mesh grill covers on my car, and one day we had some frozen rain/snow while I was driving to work.
About 3/4 of the way there, my engine started to get REALLY REALLY hot (almost in the red on the temp gauge). I let off of the gas and coasted for a while, but the temp wouldn't drop significantly (it did drop a few degrees, but not to where it normally runs at).
When I got to work, I was a little puzzled. When I was ready to go home, I noticed what was causing my car to overheat.
The entire front of the grill was covered in ice (about two inches worth). The water (as well as the ice and snow) hit the metal grill and froze, completely blocking the area. No air could directly hit the radiator.
I quickly chipped out the ice, and everything returned to normal.
well, it may be best to take it off in that situation.. however it defeats the purpose of catching the bugs and small rocks from hitting your insides. so its best if you take it off. The one i have on my car is removable in the middle and the sides are secured by special "mounts"