Burgess J.W. Raby & William L. Raby have published Painting the Accounting Practitioner Into a Tax Practice Corner, also available on the Tax Analysts web site as Doc 2005-18877, 2005 TNT 178-4.  Here is the Introduction:

Start with the proposition that tax practice standards include levels of confidence prescribed by authoritative bodies as necessary for filing returns or taking opinion positions. By the time year-end 2005 work is being done for business clients, it seems likely that standards will be in place covering (in order of increasing confidence):

• nonfrivolous positions (a level of confidence sufficient for return preparers, if accompanied by disclosure, but not sufficient for taxpayers to avoid penalties);
• reasonable basis (the level of confidence generally needed for the taxpayer to take a disclosed position without penalty);
• realistic possibility of success on the merits, assuming the treatment was challenged by the IRS (the level generally sufficient for return preparers to take undisclosed positions without penalty, but not quite sufficient for taxpayers to avoid penalties);
• substantial authority (the level of confidence needed for taxpayers to generally take undisclosed positions without penalty when "tax shelters" are not involved);
• more likely than not (both the level of confidence needed for taxpayers to take positions when tax shelters are involved, although, in the case of reportable transactions, this must be accompanied by a Form 8886 disclosure, and the standard for practitioner opinions covered by Circular 230, section 10.35); and
• probable of being sustained if challenged by the IRS (the level of confidence needed under generally accepted accounting principles, under the pending interpretation of Financial Accounting Standards Board Statements (FAS) 5 and 109, to reflect a tax asset on the balance sheet or to avoid reflecting a tax liability greater than shown on the tax return).