If I am trying to keep the bike original and it is NOT a high-end go-fast bike (read Colnago/Steve Potts or equal), then I leave them, especially the metal ones.

I also keep them on 'statement' bikes, like my 'look what a dork I am' 1985 Schwinn Sierra with painted matching fenders, abnoxiously huge bell, and coffee holder, complete with a squeeky mattress seat.