About to embark on a DIY project to build my kids a fort. It will be a deck using 4x4's for legs, holding up a box of 2x8's. Three interior joists for a total of 7 2x8's will make up the frame for the floor. The 2x8's will be attached to each other using 3 1/2" long, either 3/8" or 1/2" thick galvanized lag screws. On top of this, I will be building walls 5' high out of 2x4's and cedar fence boards, with a simple gable roof.
My question is: should I use blocking between the floor joists? I know this prevents cupping, but is there a structural need, or is this overkill?