Don't cleat it off! Poly ball the end if you feel the need, but unless you've got a real short line to your dart, you wont need the ball. Most fish almost give up once they are pooned.
Poly ball if yu can, if you must cleat off, do it to your bow. I agree, most fish will give up once pooned, and if they don't usually they make a quick circle and come right back. I would have the poly ball attached, or add some length to your dart lines to add safety. The poon site has some good tips, check them out.
tying off the harpoon is a good way to pull it out of the fish. rig t with 100 feet of parachute cord to a poly dart ball. if the fish runs out and it looks like its getting close to the ball, clip a snap swivel from another rod to the dart line and let the fish run with very little drag (just enough to keep the reel from backlashing) this way when the fish stops, you can reel the dart ball back to the boat instead of chasing it across the horizon. the ball and dart will break the fish's spirit to live
I never understood how the guys who poon swordies and then leave them to die on a poly ball dont come back to a fish that has been ravaged by sharks or other predators.