Photoshop doesn't use RAM until it runs out and then use scratch disk space. It starts using it from the beginning and it attempts to speed up your workflow by anticipating your needs and allocating the resources to it. If you're working with large files and your wife isn't, for instance, it will try to allocate enough scratch to easily accommodate those large files.
This should tell you more about scratch disks, how and why PS uses them. If all you're getting are some temp files that go away when you quit Photoshop, then you probably have nothing to worry about.