generally you will want to pay the tax from 'outside' money (taxable brokerage / bank account) not money from the retirement accounts or you are eroding your tax advantage by withdrawing from these accounts prematurely to pay tax, given all the hoops we go to get money into these accounts.
watch out for paying sufficient estimated tax during the year depending the size of the tax on the conversion, if you wait til you file your return you may have an underpayment penalty depending your situation.
