Recently I moved to the Olympic Peninsula in Washington, and let me be the first to tell you that it is a MUCH wetter climate than the one I came from in sunny California. I noticed within my first month of living here that my trusty rough out saddle learned a new trick, squeaking incessantly. I have never experienced the notorious squeaky saddle. I began my search to end the squeak once and for all.
I began by brushing the dirt and dust from the saddle, taking care to get under the fenders and bust all the dust loose hoping that would help the squeak, but no luck. I read that it was okay to oil a rough out saddle, however I have heard some horror stories of ruining saddles and was not sure I wanted to take that route. Baby powder was my next adventure, and let me tell you, it was an adventure!
My final words of advice are to do what makes you comfortable. In my searches for ways to take care of the squeak many recommended using non-darkening oil. Which I was a big fan of, until I got my rough out and was always told to not oil. However, many have had luck with oiling their rough outs. So I think it really comes down to personal preference and comfort level.