I was told that Social Security insurance benefits are not income taxed by the IRS. Is this true?
That depends. If your "combined income" which includes some of your SS benefit, exceeds a set threshold, yes, at least some of your SS benefit will be taxed (but not all). If you know you have other income, be it from investments, your work, a spouse's work, and you would like to have the SSA withhold taxes so you don't suffer paying it by April 15, you can request witholdings.
You want IRS form W-4V. When you complete the form, you will need to select the percentage of your monthly benefit amount you want withheld. You can have 7%, 10%, 15% or 25% of your monthly benefit withheld for taxes.