I've printed a bunch of these vouchers and have used them often. Whenever clients owe taxes year-end, it's the *easiest* way to adjust their withholding for future years. I fill it out for them (using one of the four % available), have them sign it, and often mail it for them to the local social security office whose address I've pre-printed. Clients hate to owe money at the end, and this is painless for them and for me. The SSA office within a week or two sends them a letter in large print confirming that they have requested this withholding, and lets them know the date and amount of next direct-deposit. I highly recommend this method.