Super Low Cost Deals: If it sounds too good to be true... It's easy to be taken in by companies offering "free" or "very low price" cleanings. Those companies are not covering their costs when they run ultra low deals. They often rush the cleaning part and feel pressure to "find problems" that they can charge more for in order to "make up the difference".
Low on Freon Trick:
If a company ever says you were short and needed a "freon recharge" ask them where the leak was. Your AC is a closed system. You can think about this like you think about your car brake fluid and gasoline. You always need to refill your gasoline because it's being used up. Freon is not like that. Your freon is in a closed system like your bake fluid. You don't "fill up" on brake fluid every time you go to get gas. If break fluid or freon is low-- there's a leak. If an untrustworthy company can't find something to charge you for they might say you were low a pound of freon. Beware.
Energy Star has a check list that you can use to help make sure that your HVAC maintenance is done well.