Looking good! Love seeing these projects done out of a garage. You might as well reinforce the seams, I can't see it adding that much weight and seems like a great idea especially for shock loading and spreading out the force on the joints.
From someone that tends to overbuild/overthink everything myself I think its worth the extra time to not have to worry about the project failing down the road, or redoing it. As as home builder/hobbyist most don't have the time or money to test parts to failure to see what you can get away with, even if you did it isn't worth it unless you are going to build multiple units.