If you have this as a sales pitch then it should be everytime, maybe just a quick brush over. I don't use it as a selling point, so they don't get done every time, but the frames are always clean, there is really any dirt on the glass after 4 let alone the frames!
I do clean the frames thoroughly on first cleans, then go over them every 2-3 months, they don't need any more than that to stay clean.