As long as you are talking about domestic ACH items, I believe that there is as yet still no mandate to screen any such transactions against the OFAC list. You need to make certain that your agreements with your Originators include a provision that they are not sending to SDNs or blocked countries, and you should of course also be screening your own account base so that you aren't doing business with any such entities. You then rely that the RDFIs are doing the same.
A brief discussion on this topic can be found here:
http://www.kirchman.com/comply/hot_issues/ACH-OFAC.html