Thank you all for the very helpful responses. I have the standard long, multicolored hydrometer that your local brewshop carries, in my case it's Bitter & Esters in Brooklyn New York. I don't remember what the temperature is supposed to be for water to record gravity at 1.000. I think it's 68 degrees. I never actually thought about calibrating my hydrometer, and will do that today. The idea of using a consistent method of reading makes a great deal of sense. Even if the reading is off by a line, if the original and final gravity readings are made using the same method, then you can determine ABV, and whether or not your wort has fully attenuated.
Thanks for the help!