that's very debatable. I see where you are going with it (sub loads off the trunk and the low end has more authority) but you can get it to sound very good facing forward as well. the downside is you hear more mechanical noise from the sub if it's distorting at all.
And you guys need to tell me your techniques on sound deadening because when I faced my fi bl at the trunk lid (expaning foam in holes and 3 layers deadening on lid, 2 layers in entire trunk) there's still a **** ton of rattle. license plate was taken care of with foam, seems like the whole rear bumper rattles even thought the fasteners are tight. might need to remove it and deaden the actual bumper