I have shot powder - wonderwad - maxiball but never saw the need to put another wonderwad on top of it. If the bullet is seated right and the right size it should not be moving around down there.

Now when I shoot my smoothbore I shooot powder - over the powder card - fiber wad - .610 diameter 343 grain roundball - over the shot card. This is necessary because I do not patch the roundball. When that big lunker hits downrange, you know it....