I used a #8 perle cotton, it seemed to suit the brighter more modern fabrics I'm using and I'm doing a stem stitch. Cosmo or Precencia is more colour fast than DMC which I've had bleeding before.
I have used 2 strands of DMC embroidery floss. The reason? I already had the colour I wanted to use. I have done stem stitch as well, but I think this latest block had chain stitch.
I'm using DMC #5 pearl cotton (the kind that comes in a hank rather than a ball) in a stem stitch. I've washed each block when I finish it to get the markings out and haven't had any problem with it bleeding. I used embroidery thread in a chain stitch on the first one and am going to take it out and redo it with the pearl cotton stem stitch.
So far I have just used DMC floss because that is what I had in the house.
Using DMC floss - 2 strands. Like the color range that Weeks Dye Works offers, & have quite a bit of it on hand - but my supply is the older floss, before Weeks improved the color stability.