Whenever you write your family history, even if it is only intended for the family, it is important to include an index. How many times in your research have you picked up a book only to find that it lacks an index? I venture that most people scan through it quickly then return it to the shelf, deeming it inefficient to spend the amount of time it would take to search the book page by page.
Introductions should be indexed, but a preface should only be indexed if it goes into depth on the subject on the work. If it is simply a discussion of how the book came to be published, do not index it. Glossaries and bibliographies are not indexed. Only index footnotes if they contain explanatory material. Do not index notes that contain only source citations. When indexing notes, include the letter n to indicate that the user should search in the footnotes on that page, e.g. 324n.
For more information on rules for indexing, see the Chicago Manual of Style, 16th edition, 846–49.