You either plan to tell the truth or you don't. If it is your intent and plan to tell the truth, then I don't see why you would not affirm that under oath. If it is your intent to not tell the truth, then the only reason to not affirm is because you want the court to know that -- that you cannot be trusted to tell the truth.
I want to had the same issue and I was a kid. What happens if your ass to swear to tell the truth I promise to tell the truth and you say you won’t make that promise. You could be held in contempt for refusing to testify to the truth More
If you refuse to take the oath, you may be dismissed as a witness, or held in contempt of Court.