No, that is a global option. You can change error_reporting on the fly to simply stop the warning for deprecated functions by doing this.
// code for session_is_registered();
error_reporting(E_ALL & E_STRICT);
That just stops the warning for your code, then resets it to the default.
Better would be to write your own function for session_is_registered() that sets error_reporting, does the function, then sets error_reporting back.