bash scripts and files

[michael <cs@networkingnewsletter.org.uk>]

Folks, can somebody point me to an authorative reference that  
explains when one needs to put, eg,
 #!/bin/bash
as the first line of a script and whether or not it's required/ 
surplus/ignored for bash specific files such as .bashrc  
and .bash_profile

many thanks but couldn't find it quickly using 'oogle. M