I love your quilt!
I have tied with DMC embroidery floss, before, and I had to re-do them again because they loosened up after washing the quilt in the washing machine. I had tied them with the surgeons knot, too!
I would probably use crochet thread next time. My grandmother tied her quilts with yarn and they are still holding up even after more than 40 yrs--and we use those quilts year round!!!