If referring to the bolt at the bottom of the metal "tube" that runs down from the oil filter housing, it's the oil passage plug and its only intended use seems to be to attach an oil pressure gauge for troubleshooting oil pressure problems.
If no applicable maintenance reference tells you to remove it when changing the oil filter, I personally would leave it in place and not mess with it. If removing it truly served some purpose when changing oil, it would most likely be mentioned in a service manual.
If you choose to remove it, I would recommend using a torque wrench and Loctite during re installation of the bolt. It wouldn't be a good bolt to strip upon installation or have come loose while riding.
Very good question, by the way.
Last edited by planalp; 01-07-2013 at 08:24 AM.