As a general rule, no you may not. If you used a particular accounting method in reporting your income, then you are bound to that same method when challenged by the IRS, including at any subsequent trial. For example, you cannot report your income using the cash method, but then later at trial use the accrual method. Effectively, this precludes a person from using the accrual method as a defense as to why income was not reported on the return—namely, because it is less than what the IRS asserted using the cash method of accounting.