I'm no purist--meaning I use lots of different threads. I find that I choose by weight of the thread more than anything--in blocks with lots of pieces I tend to go with a higher # (at least a #50 but often a #100). For something like a rail fence block, pretty simple, I'll use a lower # weight. I know lots of folks dog on Coats & Clark thread, but I find that their thread really doesn't tangle or knot when I'm sewing down a binding.