I only refer those that I know are reliable and will do what they say at the price they give.  There are far too many companies that give low prices to get the job and then come up with all sorts of other problems that increases the price after they have already gotten the contract and started doing work.  Granted, sometimes this is unavoidable, but for the most part a contractor should be able to make a good enough judgement of the job to give a proper and accurate quote for a job and be able to get the job done in the time they estimated.