I've built 3 x 15" boxes with 8,000 watts before without screws, and it worked just fine. If I had to warranty them, I would probably use screws also. I'd be more concerned about them breaking when taking them in and out of vehicles.
...On a related note, I ran my hand through an alternator belt once, it threw the belt and killed the engine. It took 26 stitches and broke all 3 of my fingers. I still don't have full flexibilty in that hand.