"Model releases are not generally required for editorial and educational uses, such as books, magazines, etc. To "educate and inform" is the key." (Hopkins, 1995) On the other hand, "commercial use does require a release, such as [a] product endorsement, advertisement, etc. [Stock] Agencies [usually] want releases because it's too much bother to make sure your photo(s) don't end up in some kind of advertisement. And, the [money] is much higher [for] advertising and commercial use. Naturally, certain sensitive issues (teen pregnancy, drug use, AIDS, etc.) will generally require a release." (Hopkins, 1995)
As a general rule, ALWAYS get a model release if you can. You'll ALWAYS be better off having them because it will increase the salability of your images. Obviously there will be times when getting a release from someone will seem impossible, but attempting to sell such images may attract litigation. NEVER pass on the opportunity to get a release from someone if it IS possible!