I use whatever matches, would spend too much on it unless it was a competition or extraordinary piece, which I haven't done so far. Rayon thread and other ME thread does melt nicely into the edge, but my stitches aren't that visible either way. I'm somewhat suspicious that a good portion of the thread hoopla is hype to make us buy more thread. I mean, like the thing about old thread. If the thread in my clothes gets exposed to light, temperature changes, wear, and laundering but it holds up, then why won't quality thread, stored away from light and humidity, be okay?