I would say so.
If you think about the boat sitting in the water, the buoyancy is at the tubes, and the floor sort of hangs from that.
OTOH, I've seen everything from a 6 bunk trailer to a flat bed for small inflatables, so it's kind of hard to really say what's "best". (Actually, beyond flatbed, I guess: One group pulls the motor, inverts the SIB, and ties it into a larger trailered RIB to transport.)
The keel will stretch the fabric of the floor away from the decking, so you don't want support there. That leaves the keel itself, the Tube/Floor intersection, and the tubes themselves. Given that you may need to transport the boat with tubes-less-than inflated (for whatever reason), I'd go for the tube/floor joint.