I think it is reliable. and at the end of the day its better to have you packet out and processing than waiting. I'm actually going to be moving soon so I'll notify Uscis and make sure I do an address change with the post office. If it is sent to your old address it will be returned to the post office. So if you're going to be in the same general area that shares one post office you can periodically check with them to see if any mail was returned for you.

If you're still worried I would call the Uscis hotline and find out if there is a way to confirm they have received your address change or make an appointment to speak with someone at the office. I actually was able to make an appointment online and the process was quick,easy and they gave me a choice of times and dates.