If you plan to withdraw funds from a tax-deferred account, make sure to carefully examine the rules on exemptions for early withdrawals. For more information on situations that are exempt from the early-withdrawal income tax penalty, visit the IRS website at www.irs.gov....
the risk that even though you average 8% (or whatever) returns over your retirement, if the crummy returns show up early, the combination of bad returns and portfolio withdrawals will cause you to run out of money early. So the amount you can safely take out each year must be low enough...