According to this forum thread:

Yes, the turrets were manned during the Battle of Endor. As mentioned above, those shots were deleted, however. An image in the STAR WARS: THE ACTION FIGURE ARCHIVE shows the man seated behind Lando in one of the turrets.

This is confirmed on Wookiepedia list of cut scenes in ROTJ

There are cut shots of rebel gunners aboard the Millennium Falcon.

According to this forum thread:

Yes, the turrets were manned during the Battle of Endor. As mentioned above, those shots were deleted, however. An image in the STAR WARS: THE ACTION FIGURE ARCHIVE shows the man seated behind Lando in one of the turrets.

This is confirmed on Wookieepedia list of cut scenes in ROTJ

There are cut shots of rebel gunners aboard the Millennium Falcon.