This page gives you a typical wordpress load http://codex.wordpress.org/Plugin_API/Action_Reference
You can see, the current user is available after the theme is setup but before init. If you are calling these functions within the theme then you will be fine. Otherwise make sure any functions that reference the current user are called using hooks that run after the set_current_user hook. The best hook to use will vary depending on the specifics of what you’re trying to do.