The five pointed star, for a long time, was symbolic of the five wounds of Christ. However the pentacle is now mostly associated with Neo-Paganism (such as Wicca).
An inverse cross originally was simply representative of St. Peter, as others have mentioned, it was St. Peter's cross just as St. Andrew's cross is an 'X' shape. The Scottish saltire is a St. Andrew's cross, as St. Andrew is the patron saint of Scotland; and as such also forms part of the flat of the United Kingdom (the flags of England, Scotland, and Ireland or more accurately the Cross of St. George, the Cross of St. Andrew, and the Cross of St. Patrick superimposed):
In any event, the point being that just because later on a symbol similar to St. Peter's cross came to be used as a symbol of Satanism and adopted by modern Satanists as an intentional inversion of the Christian cross doesn't change the historic meaning. Neo-Pagans and Satanists don't get to dictate which Christian symbols we may or may not use.
As a good example, the swastika has a very long history in the Vedic religions, among Hindus and Jains. Should Hitler and the Nazi's perversion of the swastika mean that Hindus and Jains can no longer use one of their own religious symbols?
-CryptoLutheran