Look online at the AM or FM self-inspection checklists. There are NO regulations about restricting studio access, locking doors, or anything of the like. The 2009 self inspection checklists are the latest available, and considered to be the same rules under which you would be liable for violations. You can be as liberral in being public-available as you are comfortable-with, and as conservative as you want, given access is somehow available to a public file during REGULAR BUSINESS HOURS which you also can set within reason of available staff.