Section 6654(g) of the tax code indicates that withholding from all sources is by default treated as withheld evenly throughout the year. However, it does allow the tax payer to choose that withholding from wages be treated as payed when withheld (the same as the treatment required for estimated tax payments), separately from the choice to treat withholding from all other sources as paid when withheld. RMDs, as is any income reportable on some type of Form 1099, are a source of income other than wages from which taxes can be withheld.
