Yes, any automotive compressor leaks at the shaft seal, but if it is leaking at a rate that requires annual charging, the leak is too profuse and must be corrected.

A typical system will lose its charge in about 10 or 12 years. If you are charging it as often as once every year or two, you should consider this a leak, find it and repair it.

Auto a/c compressors don't leak any more in the winter than they do in the summer. In fact in most compressors they are more apt to leak at the shaft seal while the compressor is engaged and cooling. You might be getting this mixed up with the fact that the compressor shaft should turn a few turns at least once per week in the winter to keep the seal in good shape.

Have a great day,