Michael, this is a pretty subjective area that depends a lot on your lifestyle and criteria. There are a couple of factors that go into making your decision.
Since the house has been vacant for a year and a half, there has been no human or pet activity in it to speak of. The activity generates the dust that you want to remove.
Has the furnace been on for this time or were the pipes drained? If the furnace was off, you probably did not have significant amounts of air moving through the ducts. If it was on and the furnace filter not changed, how much dust is in it? This will give you a great clue.
Do you have anyone in the family with an allergy problem? Are there signs of water leakage in the basement or crawl space? High humidity means that mold and mildew could be growing very nicely in the ducts.
Answering these questions should give you pretty good direction on how to proceed.