As Tartan said the flat metallics like Sliver tend to twist if you use a horizontal thread pin. I've used Sliver on my Bernina 180 with the vertical thread pin but I do have to loosen the upper tension.
Feline Fanatic, what Bernina do you have? I don't know about the new ones, but the older ones often had a vertical thread pin that you could get. I bought one for my 180 and use it as my regular thread pin.