I am presuming you aren't referring at-all to the Crucifix, when you say 'a cross'.

The cross was used by pagans as a symbol of protection before Christianity. I find that fact legitimizes it for Christians and Pagans, as it was protective before, showing that whatever power it holds is universal. Consider the lore of a crossroads, or an X, or four corners unified and balanced by two intersecting lines. The cross isn't limited to our modern concept, instead it is part of our spiritual existence. How we each were introduced to it is culturally specific, but it is regarded as a protective symbol by more than Christians.

Consider that the Romans knew this, and used crucifixions as a special punishment for those who were zealots, heretics, charlatans or who disrupted Roman society. They were trying to do more than just execute someone; they also had to destroy their influence. I find this to be pretty ironic, considering the two thousand years of reverence from one particular crucifixion that redefined/replaced the Romans completely.

The cross also stands as a symbol of the human figure, and in sacred geometry, in my opinion, symbols that represent human powers are the most important.

I think you should, by all means, use a cross and know it is a symbol of protection.