I was taught that hearing people are given name signs by Deaf people as a sign of acceptance into the community, and aren't supposed to make up their own. (I'm sure it's easier in a classroom situation to make up a temporary name sign for each student though.)
There is a history and system of Deaf name signs in ASL, and there is in fact a book about this called "The Book of Name Signs" by Sam Supalla. My name sign (given to me by a Deaf friend) certainly doesn't follow the rules in this book and I've seen many others that don't, but it's good information on ASL linguistic tradition.
Here's a link to the book -- I am not affiliated with Harris Communications, they just have a great, extensive catalog. If you click on "view chapter sample" you can actually read part of the book which may further answer your question.
http://www.harriscomm.com/catalog/p...=2126&osCsid=23b4fdbb30070090516abd2820991a49