"Note that I don't recommend hooking the add_custom_rewrite_rule into the init action as it will fire too frequently. This is just an example. A better place to apply the function would be on theme activation, plugin activate, perhaps the save_post action, etc. Depending on what you need to do, you may only need to fire it once or just a few times."
I found this recommendation here. In my WP installation I use a third part plugin that is hooking into the init action to change a custom post type slug. Is this a problem?