Debbie,
I am a licensed dental hygienist and the recommendation is that you have a panorex or full mouth series of x-rays every three years. The panorex shows parts of your anatomy that are not seen on the standard intaoral full mouth series of x-rays. Most of the dental practices I train (probably over 100) alternate between a full mouth series of x-rays and a panorex every three years. Your insurance will likely pay for it, but you can call and ask them. The ADA code for a panorex is D0330, so give your insurance carrier a call and ask them about coverage.
hope this helps, Barb