Been doing some reading on the subject and a few things stood out that I haven't thought about.
In the days of $39 uppers, tolerences can be all over the place. You may very well end up with one so out of spec, even the tool won't fit down inside.
If you can true it up in a lathe after centering on the rails is the best way, otherwise it is possible to apply to much pressure the wrong way.
Some claim that your sights will be centered. Others said it's easier on your bolt. Others said it was a solution looking for a problem. Most said it doesn't hurt a thing but not a single case of "it turned my blaster into a hammer". Most cases the amount of material removed is about 1/1000". I have a stripped upper from Rainier Arms here and pulled a carrier out of my Colt 6920, the carrier to receiver rails moved up and down and sided to side well over 1/1000", so did the bolt inside the carrier.
Now that said I will relay a story told to me by a friend who had a rifle that shot well but not great. He knows a famous shooter/gunsmith and they started talking about said gun. The guy took it and turned it into a great shooter. My friend said he didn't see what he did but he told him the bolt was not square so he lapped the bolt. Maybe he lapped the upper? I truly don't know, he just said he lapped the bolt.
I still say there is too much tolerance in any upper/carrier fit to make a difference in most but I'm gonna say if done right I suppose it won't hurt a thing either.
