Is it safer to use a guest account when streaming from projectfreetv?
In this Top 10 list (for Mac Internet security), one recommendation is:


  Special bonus item (for advanced users):
  
  Consider running your Mac as a non-administrative user most of the time. This is an advanced technique that provides few immediate
  benefits, but, similar to anti-virus applications, acts as "insurance"
  by limiting any damage that could result from a security breach.
  Someone gaining access to your machine would still have access to many
  of your files, but not to most of the underlying machine and OS
  itself. The easiest way to run as a non-admin user is to create a new,
  administrative account and then change your normally used account to a
  non-administrative one.


This step would be part of a layered security approach, and probably could be applied to any OS - one wouldn't normally use an admin/root account in Linux, for example. Other sources endorse this approach as a security best practice (see this forum post and this page from the Stanford Secure Computing site).