Preventive maintenance on items in your house saves you a lot of money in the long run. Simple PM items can be done by the homeowner such as cleaning vents, caulking window seals and etc. Not taking steps and having to call someone over for "emergency service" is when things quickly get expensive. So if you feel like your unit might not last the cold winter months get it checked out in the fall, because waiting until peak season will cost much more for the exact same service.
Overall the message in this post is be pro-active not reactive!