Since a Roth IRA doesn't force you to ever take out money, you can dip into this last. If, however, you look to be in an abnormally high bracket for the year - say, because you took a job - you may want to withdraw from your Roth IRA first.
A Roth IRA can be a nice way of giving you more flexibility in managing your income in retirement. If, for example, withdrawals from a 401(k) combined with Social Security and other income are about to push you into a higher tax bracket in a given year, you can always dip into your Roth.
It's also important to remember that your Roth IRA can be a great means of harvesting tax losses. By selling an investment which you've lost money on and applying that loss against future capital gains, you are, in effect, reducing your tax bill.