Go outside and check on your outside unit every now and then when there's a storm .
A friend of mine just cost himself a big chunk of change because he didn't .
Its been freezing rain / snowing here since about 10 am
About an hour ago I got a call from my less then mechanically inclined friend panicked because his outside unit was making a terrible racket and he didn't have heat any more .
The fan on it must have had a good inch of ice built up on it , which of course threw it out of balance and pretty much shook the whole unit apart .
Ancillary to that the whole thing was packed full of snow and ice greatly reducing its efficiency before vibrating its self to death .
Check your unit every now and then ... And a little WD 40 or Pam spray on the fan blades goes a long way to helping ice not build up when you know its possible .