The condensing unit is made to be out doors and as a result is subject to all the weather has to throw at it. Covers are great to protect the paint job and keep stray leaves from fallins inside. Your answer is "It wouldn't hurt". Tarp is fine but just remember to take it off next spring or trouble will follow.
I concur with the above. They are made to be outside. Of course my Carrier decal on the top is faded beyond recognition in only 2 years but it doesn't affect the operation so who cares?
I have always told people that must cover the condenser to only cover the top. That keeps the big stuff out and lets it breathe. I have seen condensers covered better than food in the refrigerator. With the temperator differances seen thru-out the winter, you have created a rust accelerator when you wrap them up that tight.
Don in Mo
As mentioned, this question comes up time and time again. I don't think that I can remember a year that this question was not posed.
As the others have mentioned, the unit is made to be outisde, so covering it is not mandatory. If you are concerned with leaves and debris entering the unit, then by all means, cover it.
If you think that by covering the unit you will be extending the life of the system, think again.
Have a great winter!