IMO it is best to have income you can count on in retirement. In other words it is best not to plan on drawing out any of your principle savings but ideally should be able to live off of the income that it produces along with any other retirement incomes like SSI, pensions etc.

Now exactly how much income is personal matter based on needs and lifestyle, but best to be able to count on it. The worst thing IMO is for a person to continually draw out of his principle in retirement and then out live it and yet still need it.