Parallels Desktop only allows you to enable or disable USB emulation, which likely is the equivalent of enabling or disabling the emulation of the USB host controller.
After that, you can only decide which attached USB devices should belong to the Mac OS and which to the virtual machine.
Actually, one can't even assign real USB devices to a virtual machine before one enables USB support in the virtual machine emulation. So from that point of view it's fool proof. Of course, that doesn't guarantee it's bug free…