John L. Guyton (IBM Business Consulting Services), Adam K. Korobow (IBM Business Consulting Services), Peter S. Lee (IRS) & Eric J. Toder (Urban Institute) have published The Effects of Tax Software and Paid Preparers on Compliance Costs, 58 National Tax Journal 439-48 (Sept. 2005). Here is the abstract:

In recent years, the percentage of individual taxpayers using paid preparers and software has increased, while the share of taxpayers who self–prepare without software has dropped sharply. Using the Individual Taxpayer Burden Model developed by IBM Business Consulting Services for the IRS, we simulate the effects of preparation method on time and money costs of preparing tax returns. When we correct for self–selection bias, we find that each group on average selects the preparation method that costs the least for them. For example, software costs more than self–preparation for current self–preparers, but costs less than self–preparation for current software users.