As long as you have finishes in your home that you love and that aren't too over-the-top-trendy, I say go ahead and mix them up. There are no hard and fast rules any more. Keep it simple but add in things that bring meaning and pleasure - no matter what kind of metal they have on them.
Knock on Wood Design, via House & Home
Hecker Guthrie via 79 Ideas
photo by Michael Graydon