Please make your world(s) believable, Electric Spec writers. If your characters are going around in their shirt-sleeves, make sure it's a habitable planet--unless, of course, your characters aren't human. :)
An excellent free reference for world-building is "Habitable Planets for Man" by Stephen H. Dole, the famous RAND Report from 1964. This 169-page (!) document was prepared for the U.S. Air Force and includes such things as Human Requirements, Probability of Occurrence of Habitable Planets, The Nearest Candidates, and even a nice glossary. Granted, some of it is dated, but it's all fascinating. Enjoy!