Do you know what kind of cedar it is? Is it a Thuja species? It probably would be ok but it would be good to know what type it was.
All that would matter is that they won't grow back out again much, if at all until next spring. I suspect some property owners with large hedges deliberately have them done in winter with the idea that they won't have to repeat the expense for some months, this being more of a priority than having them releaf soon after pruning (and look better, to those of us that think so).
You only have to give the plant time to seal up the cut so if you know you have three days frost free coming, you will be fine. Cedar hedges seem to take the best appearance pruned in late june and in early september giving two fresh flushes each year. Also minimizes the size of the pruned pieces. imo.
I agree with Greengoose. I try and get to my clients hedges after the spring growth spurt and again after the smaller growth flush in the late summer. You can do them at other times but there are trade-offs such as frost tenderness to newly exposed vegetation, lack of regrowth till the next growth flush (to cover any oops cuts or imperfections) and if you do them just prior to a growth flush, you gotta do them again afterwards anyways if you dont like them to look a bit shaggy..
