when to use include (inc) files in module development
As general rule, I would put in the module file the code that is required more often (e.g. helper functions used from more than one function), and in .inc files the code that is not used so often, or that is used for specific pages.

Since Drupal 6, the code automatically loads the files containing the page callbacks, or the form builders used for the menu items. For that reason, page callbacks for administrative pages are normally put on .admin.inc files, while page callback for normal pages are put on .pages.inc files. 

Since Drupal 7, files containing classes are automatically loaded when a class is instantiated. Drupal 7 allows modules to define in which files their hooks are defined (through hook_hook_info()). For example, system_hook_info() defines the .tokens.inc files as files where hook_token_info(), hook_token_info_alter(), hook_tokens(), and hook_tokens_alter() implementations can be found; in this way, those files are automatically loaded when one of those hooks are required.
This allows to further split the code in files that are loaded when necessary, and code that is always loaded from Drupal.