I have never heard this before. So what if you are planning to go out of US after OPT expires?  I think your lawyer is BSing. USCIS doesn't care if you WILL be out of status, they only care if you ARE out of status. They don't have time to look into all different aspects to make sure you are always in status that is not their priority. However if you are not in compliance it may affect your future application. My suggestion get second opinion from another lawyer. You think these lawyers know everything, you will be surprised how little SOME of these idiots know.