The BBB conducted a study a few years ago and found something really interesting. Out of the 10.7 Million BBB complaints, 80% were the result of homeowners choosing the cheapest bid on their home improvement projects. They found contractors no longer in business when a warranty related issue needed addressed, contractors failing to honor warranties, improper installation, taking down payments, damaging personal property, etc. They typically focus on quantity and not quality, which justifies them selling the job cheap.
Just because a roofing company has the cheapest price does not necessarily mean they will provide a bad service. But, if one company is significantly cheaper than your other quotes, this is something to take a second look at. Companies who are significantly cheaper are very likely not installing your roof to manufacturer specifications nor local building codes (how would you know?). If they are the cheapest price and installing the same shingles, they have to be saving money somewhere… These companies save cost by having bare minimum insurance coverage (if they have it at all), using cheaper, non-compliant roofing underlayment and roofing accessories, and not hiring a quality control manager. When recession time hits, guess who is most likely to be out of business and unable to honor their warranty? The companies who are the cheapest.