Running the line forward instead of laterally increase the 'prevention' - my boom can not shift aftward at all. Running the another line aft allows me to fine tune fore and aft.
As rigged, the boom simply can not move irregardless of what the wind is doing.
I also trust my cleat more than my stanchions/toe rail to handle the load of 340 sq feet of mainsail (about 600 pounds at 20 knts)- more when you figure the shock load). PCI says they are back plated/safe, but I've seen the underside (don't ask me why, it's embarassing) and they are NOT. They have nuts, washers, and lots of 5200, but not plates.