My main focus in Golden Age is Supes, Action, and Nedor Schomburg war covers. I can honestly say that it's rare with my focus to find books priced under guide. In fact, CGC 6.0 and higher Action war covers and Nedor war covers almost always bring over guide prices. If it's an especially good cover, or a Hitler cover, you could be talking mulitiples of guide. In the case of Nedor, a graded copy is very often the ONLY graded copy, so you have to grab it when it comes along, and that often means paying over guide prices.
I also have some concern about the market in twenty years or so. I think mainstream titles will be a safe investment. I'm not sure about the Nedor books, but they're so beautiful that I just can't help myself.