I would not touch, file or grind ANY spots on the main body of a piece... meaning, for example, on the inside surface of a skillet, or the outside or bottom of a skillet.
There are normal and expected casting irregularities that occurred. To alter those, would, for me, devalue a piece.
It is not possible to replicate the equipment and usage of that equipment that foundries of yesteryear used to finish their products.
The large grinding wheels, with appropriate grit, the large industrial tumblers with tumbling jacks, etc.
Stephen, if you could clarify your question a bit further, it would be helpful, as I was also interpreting that you may have been referring to old gatemarks on a piece.
There again, if a gatemark or sprue is present, it is important to preserve those in original, as cast condition in My Book... :)
What I am hearing discussed on this thread are handle areas. I do not have any pieces that have made me feel the need to consider grinding or filing an edge or a handle eye.
So, I am one who advocates the "Leave It Alone" minimalistic philosophy. (In most cases) regarding the original casting condition.
When it comes to restoration, Cast Iron pieces have more latitude than many other items classified as 'Antiques' within collecting communities. It is acceptable to remove the years of crud and gunk and oxidation to preserve the piece. (There again, principle of the least... the lesser the better... to PRESERVE the condition, not alter it)