I was told that was the respectful way with which to write it....
Unless a story is overtly religious in nature, any usage of God's name is probably not a respectful, direct reference to Him, but rather of the "Oh my G-d!" variety. So as other profanities like sh-t and d-mn it is written as G-d. Also there is a convention in the Jewish faith that states once you write the name of God on a piece of paper, that paper becomes sacred and can't be thrown away or destroyed. Then there is the PC reasoning that any mention of God in a public forum may be offensive to some so must be avoided at all costs.
I believe that here in the Religion forum it is proper and respectful to use God.