I've read this post, and I don't understand the answers to the question. I know that I'm a novice when it comes to routing, etc., but why does it matter which way the fence is moved for the second cut? It seems to me that, as long as I'm feeding the stock through the table in the correct direction (right to left), it shouldn't matter which way the fence is moved.
For example, suppose I cut a 1/2 inch wide dado in the exact center of a board, then I want to widen it by 1/8 inch, and I don't care that the resulting dado won't be exactly centered anymore. If I move the fence 1/8 inch in either direction and feed the stock from right to left again, won't it be for all practical purposes as if I'm cutting a dado for the first time? The router bit and table should act exactly the same as if I'd never cut the first dado, but was simply now cutting a 1/8 inch wide dado for the first time.
I don't understand why there would be a problem no matter where the fence was, as long as I fed the stock from the correct direction.