New Hampshire law permits a competent adult to nominate a person whom he or she would like to have as a Guardian. The statute also permits you to specifically exclude certain people from that role.
Guardianship involves the filing of annual accounts with the Court, with the accompanying expense. If you have executed a Durable Power of Attorney for Health Care and a General Durable Power of Attorney, there should be no need to appoint a Guardian. However, for peace of mind, some clients wish to execute such a document.