In Matthew 5:34, Jesus says to not swear an oath at all, either by heaven, for it is God's throne. James 5:12 also says not to swear an oath. Not by heaven, or earth, or anything else. Simply let your 'yes' be 'yes', and your 'no' be 'no'.
What are Christians to do when they are called to be a witness in a court of law? The bailiff will ask if you swear to tell the truth, the whole truth, and nothing but the truth (and sometimes add "so help you God").
I've often wondered what would happen if I were to say, "No. The God on whom you want me to swear says not to do it. I will not swear, but I will let my yes be yes, and my no be no"
That's an awesome question. I would love to see that exchange.