tell tales...
My new main came with nylon ribbon type telltales sewn to the leech end of the top three batten pockets. I wouldn't put the yarn type near any type stiching; they will hang up in light air. After reading a little from Tom Whidden's "Art and Science of Sails", I slapped a couple pairs of the stick-on yarn telltales near the draft of the main. Locate them far enough back so the mast turbulence won't affect them - 18-24 inches or so. Anyway, I think it has made an imrovement in my mainsail trim. Regarding the stick ons, they work well, just make sure the sail surface is clean and dry during application.