Use Safely Remove hardware' drill every time you want to unplug USB device
For devices like mouse, keyboard, scanner, printer you can just unplug them if you are sure they are not in use. Incase of USB device, use of Safely Remove hardware' drill is recommended so that you will not unplug USB device during data transfer. In this case you can suffer data loss. But if the device is idle there is no any need to use Safely Remove hardware' feature. If you have set USB Flash as a Ready Boost device in Windows Vista, you have to safely stop it before unplugging.