I purchased a new AR15 recently and was told by someone that the bottom of my bolt should be polished.  What good does it do or is actually a bad thing to do?

http://myweb.cableone.net/wdavis/bolt.jpg

Here is a pic that I took earlier.   Nice and new!

Unless it's real rough leave it alone. Any removed material changes headspace and if it is not done perpendicular to the bore could cause problems.

if it aint broke, dont event think about fixing it!

are you talking about the bottom of the carrier?    There is no need to polish anything on the carrier group.  Like the man said   "if it aint broke - don't fix it"

Quoted: Unless it's real rough leave it alone. Any removed material changes headspace and if it is not done perpendicular to the bore could cause problems.

Though he used the term "bottom of the bolt", I think he's talking about the bottom of the bolt carrier, since the bolt itself has no "bottom".  As such, changes in headspace, or what ever else your note seems to be trying to convey, would not be an issue.

While it is no problem to polish the bottom of the carrier, it's an exercise in futility to do so. It'll "polish" itself by rubbbing against the top edge of the hammer after a fairly short while.  In other words, a waste of time and energy.

someone that the bottom of my bolt should be polished

Let's get our terms straight... what you are showing in the picture is the BOLT CARRIER, not the bolt.  The bolt is the rotating part plugged into the front of the bolt carrier.

No need to polish anything.  Any roughness will wear smooth in short order.

Any removed material changes headspace

Not on the bolt carrier.  Does not change headspace in the least.  Only the bolt and barrel assembly determine headspace.