"Simply choosing to make the entire contribution non-deductible should always use the 8606 and never the worksheet."
I agree, but TurboTax doesn't seem to conform to that reasoning. I think I opined about it to Product Quality long ago, but probably never reported it as a bug since there seems to be some method to TurboTax's madness, just one I couldn't comprehend.
The other case where TurboTax shouldn't be using the worksheet but apparently sometimes does is when the MAGI falls entirely above or below the deduction phase-out range. The only time that TurboTax should be using the worksheet is if not all of the contribution is explicitly designated as nondeductible and, in calculating MAGI, subtracting the portion of the contribution not explicitly made nondeductible would result in MAGI falling in the phase-out range. The last time I looked at this it was pretty clear to me that TurboTax did not follow this logic.
