I advise my clients that withholding on Social Security is the last thing they want to do.  It's too difficult to make changes, or at least that has been my history with it.  I also advise them to lock their online SS account, because it seems too easy to hack.  If they have no other source for withholding, I suppose withholding is easier than quarterly payments, although those other sources may fluctuate substantially from good markets to bad.  

Since it's only name, address, SSN and choice of rate, it shouldn't be difficult to add to the program.  Nor should it be difficult to fill one out with pen and ink, if you have a steady hand.