The What-If Worksheet in the previous year's TurboTax will produce no better results than the partially functional initial release of the current year's TurboTax. The only benefit to using the What-If Worksheet of the previous year is in the case where one expects most of the previous year's income deductions and credits to remain the same and can be copied to column 2, 3 or 4 of that worksheet. Otherwise, it's easier and in some cases can produce more accurate results if the entries are made in the partially functional current-year TurboTax since the partially functional current-year TurboTax can already do various calculations that would have to be done by had when using the previous year's What-If Worksheet.
Access to the previous year's What-If Worksheet also assumes that the user has access to the previous year's CD/download version of TurboTax. It would make no sense to purchase the previous year's CD/download version of TurboTax only to use its What-If Worksheet since the partially functional CD/download version for the current year can already do everything that the previous year's What-If Worksheet can.
