The advantage of machining it is that you can take off the high spots and leave the lows in order to maintain your tolerance. It's not the best, cosmetically. But it's a great compromise for some cosmetic improvement and maintaining your tolerances.
Another viable option is cutting a groove over the bad surface, and leaving material at either end to maintain the spacing. You can actually go pretty nuts with this: Spider! does his valves finned.
Meanwhile, sanding takes material fairly evenly off of all the dimensions it touches. Which means that you're just as likely to lose your tolerance, but you have less control over whether you do.
My advice? Take it to an expert machinist and have them do it. Should cost around 30 bucks... And it's the most elegant and most accurate solution.