When I first started FMQing, the thread mattered a lot. I could barely get a good stitch with "good" thread. Now though, after a few years of quilting, I'm more likely to pick the thread that has the color and sheen I'm looking for and I can usually get pretty good stitches. I find that I can now use thread that at first I thought "didn't work in my machine". (Really it was all about how I was moving the quilt, and over time I've gotten better at it.)