it would be best to ground it to a solid point like a seatbelt bolt. or at least the chasis. there is just to much high resistance stuff between the trunk and the battery(like rubber bushings for smooth trunk operation), and could keep the amp from even working.
just remember-- a solid installation will rarely need to be touched again for service, unless you need to upgrade parts. it might be more of a hassle now to ground at the seat bolt, but it could very likely keep you from tearing your car apart every weekend when your amp starts acting wierd form a bad ground.