This question got asked couple of time already related to SharePoint 2013 and SP1.
Following blog post cover all different scenarios for you to consider. e.g. No SP1, bad SP1 and Slipstream SP1. I would highly recommend everyone to go over the following blog. It also has all the links to download SP1 for SharePoint 2013.
One thing I would like to add on top of the above blog post is that just for the curiousity I have tried installing newly released SP1 on to of my MSDN SlipStream version of SP1 and it still worked. Though I would not recommend you to do that but just in case if you installed it for whatever reason then you should not see any issues as far as my testing goes. Always verify any patches in non Production environment before installing in Production and Backup is the key before touching Production. 🙂