Hey Redbanks, I just asked this very question a couple days ago. Some say it will scatch but don't worry about it and others say they have used tank bags for years and no scratches. I for one like to keep my stuff looking as good as it was the day I brought it home. So there will be no tank bag in my future. Instead, I bought myself a really nice back-pack to wear. Hell, with the side bags and the two Kuryakyn bags I use for traveling, if I can't fit all the stuff I need, I better stay at home or drive my truck. If I did this right, you can check the link below.
Tank Bag