Example sentences from the web:

• No one knows which of the candidates Joan will vote for. She is sitting on the fence.

• I am sitting on a fence here, to go or not to go?

• Many customers are still on the fence waiting to see if a better, less expensive computer will come along.

As you can see in the last example sentence, a variation of this idiom is “to be on a/the fence“, with the same meaning.

Examples from the news:

• Bond Market Just Sitting On The Fence – (New York Times, August 19, 1995)

• In fact, I would think that he would get even more votes – from people who were sitting on the fence before […] (Yushchenko sitting pretty, BBC News, December 4, 2004).

• “It is not a question of sitting on the fence,” he said. “This is an important decision and it’s important we get it right.” (Simon Wright in a corner on tuition fees, BBC News , November 3, 2010).