Well, I found this:
http://www.dhs.gov/interweb/assetlibrary/CISOmbudsman_RR_17_Immigration_Postal_07-29-05.pdf
It's a recommendation but I am not sure if it's enforce...it depends if the envelope does not indicate the mark of return to sender. If the mail does not indicate that mark then the mail should be forwarded.
Anyway, you must put your AR-11 because USPS will stop forwarding your mail sometime and you must follow the instructions, you never know when those will be enforce or for whatever reason come up in the future. Plus, all the things through the USCIS will be considering your old address.