The law requires that a minimum of 20% be withheld on the taxable portion of a distribution that is eligible for rollover. That's why you have no option to decline the 20% withholding when you receive a check for the interest. The only reason I can see that someone would actually want 20% withheld for from the nontaxable portion would be if they had under-withheld earlier in the year. They could then complete the rollover of that portion, if that is their intent, by substituting other funds.
Most people would decline the option to withhold taxes on the after-tax portion and would deposit that portion into a Roth IRA as a rollover. Gains in the Roth IRA will be tax-free once the Roth IRA is qualified and the amount rolled over could be taken out at any time free of tax and penalty. In other words, they save the money in a Roth IRA until needed. Of course there is no point in rolling over to a Roth IRA nay portion that you need to spend immediately.
