At the higher level concours showing previous won trophies is not allowed or considered "good sportsmanship". I guess it's like showing off!
But when it's allowed, and done tastefully, I think it's ok. Just don't put every little trophy you ever won out on display. Put a few of the nicest ones, arranged artfully in a display.
Ideally, I would only like to display awards won previously at that same event in previous years, if possible.