Sentence for conspiring to defraud the United States of tax revenue, in violation of 18 U.S.C. section 371, and on three counts of aiding and assisting in the preparation and presentation of false income tax returns, in violation of 26 U.S.C. section 7206(2), is affirmed over claims that the district court erred in: 1) enhancing his sentence under U.S.S.G. section 3B1.1, as an "organizer or leader" of the tax fraud scheme; 2) enhancing his sentence under U.S.S.G. section 3C1.1, for obstruction of justice in providing falsified documents to the grand jury; and 3) imposing a sentence that did not serve the need "to avoid unwarranted sentence disparities among defendants with similar records who have been found guilty of similar conduct," 18 U.S.C. section 3553(a)(6).