Can an 2.079% APR be advertised as 2.07% APR? Or, must it be advertised as 2.08% APR?

You need to read the commentary to 1026.14(a) - Comment 2, if this is opened-end credit. For closed-end there is nothing similar - you just have to worry about whether or not that would be deceptive. .

Discussions of the "annual percentage rate" in Sections 1026.14 and 1026.21 are the closest thing to a definition of the term. Included in these discussions is the standard 1/8% (0.125%) accuracy tolerance. If I'm arguing the accuracy of your advertisement with a regulator, I would insist that the +/- 1/8% tolerance applies to all quotations of the APR, including advertised rates. Internally however, I would never allow lenders or marketing to round down for ads and oral quotations. As Randy rightly observes, it's best not to flirt with something that can be considered a deceptive practice.