If there is no power whatsoever, while not scanning, it's probably ok
to leave it hooked up. But if it has some power to it ( like a ready
condition for fast scanning ) I would remove the USB cable from it.
I've never had one like yours, since all mine were also plugged into a
AC outlet, besides the USB cable. The one I'm using now is an Epson
Multi-Purpose printer, which has a scanner on the top. Even though the
printer will not print anything ( the ink is out ), the scanner still
works.
Paul