The constitutionality of such requirements is uncertain. The Supreme Court in US Term Limits v. Thornton and Cook v. Gralike prevented states from adding qualifications for congressional candidates through ballot access requirements. If those cases applied here, it would be tough to argue that laws requiring presidential candidates to produce tax returns are constitutional as they would be adding to qualifications. However, those cases did not involve presidential elections, and perhaps state legislatures have much broader power under Article II. I think it is an open question.