You could do that, or create a class to hold your functions and make it available as a "service class" using:
<?php
$modx->getService('myservice', 'MyServiceClass', '/path/to/class/', array('param1' => 'value1'));
Then you can access your methods by simply making this getService() method call in any snippet and accessing your service object like:
<?php
$modx->myservice->mymethod();
getService() always returns an existing instance if it’s already been called, so if you call it multiple times from multiple snippets in a single request, you will only be loading the class once.
MODX Staff