I've found that a fixed number of edits, like 3, and charging a fixed price for each edit after that is a fine policy. It cuts down on the nitpicking while still allowing for customers to ask for changes. Not allowing major edits at all after the initial WIP unless it was an oversight on the artist's part (like something on the reference was missed) seems okay too. (Minor edits are more reasonable, because going from sketch to lineart can really change a piece's look.)