Refusing to pay taxes (or parts of taxes) used to fund wars seems like a logical extension of the Peace Testimony, but war tax resistance does not seem to be well supported by the teachings of Jesus, and might actually be inconsistent with those teachings.
When Jesus was asked about the payment of Roman taxes, he asked for the coin used to pay the tax (a Roman coin) and then asked whose image was on the coin. When told it was Caesar's, Jesus replied that we should "Give therefore to the emperor the things that are the emperor's; and to God the things that are God's." Matt. 22:15-22 (NRSV). (See also, Mark 12:13-17; Luke 20:20-26.) That story might or might not be a commandment that we should always pay our taxes, but it certainly doesn't support the idea that we should be refusing to pay taxes. The Roman Empire was an empire built by military conquest, and the taxes paid to Rome supported that military. If Jesus had qualms about paying taxes to support military conquest, he certainly didn't show it there.
Other teachings of Jesus are similarly inconsistent with tax resistance, even in those passages often quoted to support the ideal of pacifism. In the version of the "Sermon on the Mount" in Matthew, Jesus said "Do not resist an evildoer. But if anyone strikes you on the right cheek, turn the other also; and if anyone wants to sue you and take your coat, give your cloak as well; and if anyone forces you to go one mile, go also the second mile." Matt. 5:39-41 (NRSV). My understanding is that the "forces you to go one mile" was a reference to a Roman law or custom that required Jews to carry for at least one mile the baggage of any Roman soldier or official who might command it. Needless to say, this was greatly resented, and yet Jesus said that if your hated Roman oppressor forces you to do something, do even more than you are required to do. If our own government is evil, should we resist? According to Jesus, it is acceptable, even preferable, for us to help officials of a government that is violent.
Jesus also said that "you shall not murder" is not enough. "But I say to you that if you are angry with a brother or sister, you will be liable to judgment...." Matt. 5:22 (NRSV).
What sense are we to make of these passages? Why would God want us to help our enemies, and why would God not want us to be angry?
The common thread in most of the teachings of Jesus, and the theme that allows his teachings to make sense, is spiritual peace. We are to give up anger, and be willing to love and help our enemies, because that is what will give us the greatest spiritual peace, happiness, and contentment. Eliminating war and violence may be noble goals, but that is not what the teachings of Jesus are about. The purpose of Jesus's teachings is to help us find inner peace, and not necessarily outer peace. (Jesus said "Do not think that I have come to bring peace to the earth; I have not come to bring peace, but a sword." Matt. 10:34 (NRSV).)
A person who opposes war and other forms of institutional violence could find it spiritually disturbing that his or her tax dollars are used to help pay for war and weapons, and so war tax resistance might be more peaceful to that person than paying the taxes. But war tax resistance can have consequences, such as tax penalties, interest on both the taxes and the penalties, and even jail. (Willful failure to pay a federal tax is a misdemeanor punishable by up to one year in prison and a fine not exceeding $25,000.) And if the war tax resister has property, the government will eventually collect the taxes, interest, and penalties, regardless of the religious beliefs of the taxpayer.
So which are we to choose, the economic and emotional costs of war tax resistance or the emotional and spiritual costs of war tax compliance?
Like many other problems in life, the question seems to be addressed by the "Serenity Prayer" of Reinhold Niebuhr:
God grant me the serenity
to accept the things I cannot change;
courage to change the things I can;
and wisdom to know the difference.
Living one day at a time;
Enjoying one moment at a time;
Accepting hardships as the pathway to peace;
Taking, as He did, this sinful world
as it is, not as I would have it;
Trusting that He will make all things right
if I surrender to His Will;
That I may be reasonably happy in this life
and supremely happy with Him
Forever in the next.
War is something that needs to be changed, but tax resistance does not seem like an effective way to change it, even assuming that it can be changed. By the standard of the Serenity Prayer then, war tax resistance seems like an unwise choice.