I know its too late but I always use 2 tiedowns per side, and make knots tight up against the latch so if it does slip it can't go far. I'm extra careful since a CR250 I used to have somehow fell off the trailer and nearly overtook me whilst sliding along the tarmac at about 50mph, before hitting the opposite bank. Luckily no one was coming the other way, but the amount of metal and plastic that can be ground away in a short time is quite amazing.
Also, those bits of webbing with a steel loop on each end which go over the bars are much more secure than passing the tie-down around and re-attaching to itself. The tie-down hooks are intended to attach to your bars or steel loops, but a lot of people don't like to put the hooks on nice shiny bars.