Well, clearly you need to start tapping those accounts when you need the money to live on. But let's say you have some of your retirement stash in tax-sheltered accounts like 401(k)s and IRAs, and some of it in regular investment accounts.
Your best strategy in that case is to tap the regular investment accounts first. That way, your money in the tax-sheltered accounts will keep right on growing without Uncle Sam taking his cut. If you spent the money in your IRA or 401(k) first, you'd keep getting taxed on the amount in your regular investment account, eroding its value.