There are no rules for good photographs, there are only good photographs.
“Be prepared.” Any Boy Scout will be glad to tell you this—and to explain why it's so important if you intend to live a prosperous and productive life. And to some extent, he will be right.
Being prepared can be and frequently is a very good thing—especially if you want to be a portrait photographer and not lose your sanity at the same time. Finding yourself, for example, in the middle of a location shoot that's costing you a small fortune to stage and is far from anything that can be remotely considered civilization and running out of batteries is definitely not the kind ...