Here's my non-lawyer opinion.
If the IRS challenges you with the step doctrine, point out that a bond index fund held in a non-deductible IRA has an advantage for you. The interest stays in the account rather than being taxed, so you earn interest on interest. Although you later pay taxes, you have earned interest that would not be possible outside the non-deductible IRA.
But the most likely answer is the IRS doesn't care. Most people wait days or a month before converting, and only the growth is taxed. So if $5,000 grows to $5,024 before being converted... and you're in the 33% bracket, we're talking $8 in tax. The IRS could audit you, reverse the transaction, and hand you $8 back. Consider it from the IRS perspective - is any IRS auditor going to get promoted for finding $8 to pick on, let alone give back to taxpayers?