Moral Theologian Father Jone O.F.M.Cap. says:
"Where the tax laws are only penal laws they oblige in conscience only to accept the penalty inflicted for their transgression (Cf. 57). If they are not mere penal laws they oblige in conscience to the payment of the taxes even prior to a judicial sentence."
"Direct taxes are probably not levied by legislation which obliges in conscience prior to a judicial sentence. Customary interpretation and the intention of the lawgiver would seem to render an obligation in conscience highly questionable in our country. Although citizens should be urged to pay their share of the taxes, nevertheless, post factum, it is not necessary to urge restitution."
"Indirect taxes are commonly held to be levied as penal laws."
To my understanding, in practical terms, he is basically saying that you only have to pay what you are forced to pay after a judicial sentence. Anyting else that you can getaway without paying, is no sin.