FROM SMARTASSET WEBSITE:
This answers my question.
If you don’t keep deductible and non-deductible contributions separate, you could end up paying more taxes than you should. That’s because once you’ve blended deductible and non-deductible contributions, it’s hard to keep the two straight.
You can do it, though – so long as you keep track of your contributions. You’ll then have to divide your non-deductible contributions by the total contributions to all IRAs in your name to get a percentage that represents your after-tax contributions. You don’t have to pay federal taxes on this percentage of the growth in the account when you start taking deductions. Your after-tax contributions to IRAs are known as your basis.
You can (and should) file Form 8606 for each year that you make after-tax contributions to a non-deductible IRA. That way, you’re giving the IRS a record of your contributions that the government entity can use to calculate your tax burden in retirement.
