Burgess J.W. Raby (Arizona) & William L. Raby (Arizona State) have published Reasonable Cause, Good Faith, and Conflict of Interest, 122 Tax Notes 981 (Feb. 23, 2009).  Here is the Introduction:

When an IRS audit results in a tax deficiency, there is always potential for a conflict of interest between the taxpayer and the preparer. Once a section 6662 penalty has been proposed against the taxpayer, or if the agent is considering a possible section 6694 penalty against the preparer and the preparer is representing the taxpayer in the tax controversy, that possibility of a conflict of interest is so great that the preparer usually will be well advised not to represent the taxpayer before Appeals or beyond. In this article, we will discuss some extreme situations in two recent tax cases and a less clear situation in an older case. We then will relate those cases to the themes of reasonable cause, good faith, and conflict of interest.