I've been using a true hardwood for spacers for years with no problems or needed adjustments. Maple, Birch, and Oak are a couple that I normally use, nut never Pine, Redwood, or Firs. Of course, metal works fine, too, but it's either more money or work to get the exact sizes in metal. Weights usually end up close to the same.

Soft woods and plys will always compress after a period of time.