The simple answer to this question is, "Almost all insurance policies cover services by a chiropractor." However, the detailed answer is much, much more interesting.
In the 1970s a Pennsylvania court ruled that a large class of insurance plans could not discriminate against chiropractors. These plans had to cover services by chiropractors if they were comparable to covered medical services.
The benefit plans which weren't affected by this ruling were those funded and administered exclusively by either employers or the federal government. There are very few employers who are large enough to make taking on such a role practical, and several changes in health care coverage over recent years have made it even less practical than it was in the 1970s. Medicare, one of the largest federal health insurance plans, has had a chiropractic benefit since 1974. Limited chiropractic coverage has recently been added to veterans' benefits.
However, this does not mean that the insurance community has been forced to recognize the value of chiropractic care. There are only a few plans which cover chiropractic care because it promotes wellness. These companies have learned -- both through research studies and their own bottom lines -- that it saves them money. They know that it costs less to stay healthy than it does to treat illness.
Most plans only cover the services of a chiropractor when it can be presented as an alternative to medical care. This means that there must be a medically diagnosable condition with an appropriate treatment plan. It also means that chiropractors are being motivated to deliver additional medical services to the extent allowed by state law.
I hope you've noticed that I've referred to this kind of coverage as being for 'the services of a chiropractor' and not for 'chiropractic care'. It is still possible to get reimbursed for chiropractic care under these plans, but the services must be justified medically as well as chiropracticly.
The way that various chiropractic practices approach this situation is a very significant distinction. It also brings us to our next topic.