Of course, running Tails inside a VM is likely slightly safer than running TBB (Tor Browser Bundle) on your normal OS, depending on your threat model. For example, a Firefox exploit that breaks out of its sandbox could access your personal files when you use TBB, but only files within the VM when you use virtualized Tails (without an extra VM or network (fileshare) exploit).
If you're going to use a VM for your Tor browsing, consider Whonix [1] instead, which was developed specifically for that usecase. (Note that this is not developed by the Tor project people!)
But indeed, to get the full benefits from Tails, always use the live boot option whenever you can.
