You are not required to withhold from your social security. You do want to be sure that you are having enough withheld from your pension and employment to cover your taxes owed.
While it is not required, a good plan is to have 10% withheld from the social security for the first year to avoid not having enough withholding and facing possible penalty. If you find that you do not need the withheld funds, then you can cancel the social security withholding.
You can also use tools like Intuit Taxcaster to get an estimate of what you will owe.
