No, it isn't important at all unless someone has requested it. You can use polyester, cotton, metallic, monofilament... whatever suits the quilt and your personal preferences.
50/3 means it's a 50wt 3 ply thread. You can also use any weight you prefer. I tend to prefer midweight threads. 100wt would be extremely thin, 18wt would be very chunky. I like 30-50wt threads best.