When your AC system has been open to the atmosphere for a long period of time, it is susceptible to contamination by moisture. This moisture causes the system to not operate as efficiently as it could. In fact, moisture can really kill the operation of the AC system. Designers of AC systems know this and install either a receiver dryer or an accumulator in the system to help trap any moisture that gets left in the system. These components are filled with what is called a desiccant; it's similar to the stuff that's in those little packets at the bottom of electronics packaging. Desiccants are designed to remove moisture, but they can become saturated. Once they become saturated they should be replaced. I have heard of people removing this moisture and reconditioning these parts, but I have no experience with this personally, so I can't say if it works effectively or not.
The point is, if you leave your system open for any period of time, especially in humid climates, these components can become saturated and unusable. It's for this reason that I recommend you replace the receiver dryer or accumulator if you are going to service your AC system. It's a little bit of insurance that will go a long way to make sure your AC works as well as it can when you get it back up and running.