All that said, while I put symbols on maps, I encourage DMs to use the maps as they see fit. If you don't like the that I put a statue there, don't say it is. Or if you think a trap door in the floor would be better, put it there.
Another great resource can be found here:
http://savevsdragon.blogspot.com/2012/03/free-download-master-dungeon-mapping.html where you can download this fantastic key which is similar in many ways to the one above. New Big Dragon has a great blog and produce excellent gaming resources.