A major critieria in the immigration process is the USCIS's "perception" of the truthfulness of the applicant. Whether this preception is true or not, it is what they will be basing thier decision on. If you provide them any reason to believe that you are less than truthful in your application, they can deny it solely based on on that. Although most ommissions of information are not intentional, having to explain those mistakes later can be quite burdensome.
It is imperative that an applicant thoroughly reviews his/her application before submission. Once it is submitted, can retractions or oral modifications (at interviews) can be construed as not be truthful.