I've used the figure 9 for a couple years now on my tarp, a Crazy Creek sil-nylon (very nice) and find they make tensioning the tarp immensely easy.  They are very easy to move up and down the rope too, if your tie off is closer or farther then the last one and much quicker than moving a trucker's hitch.  When you tension, run the free end over the hook so it hangs straight down - less likely to come undone due to a breeze or gravity.  I also like to add a quick slip know to help prevent that.  Hardest problem - teaching others that are unfamiliar, the right way to use them when they give you a funny look!