On the other hand, if you're not copying flat stuff, what does field curvature matter? Sometimes in the real world I find it works for me rather than against me.

I believe in general the rule is that sticking a piece of glass in that the maker didn't design and didn't anticipate is usually not going to make things better, or if it does, it's an accident. In the case of close up lenses, what they are actually doing is turning the lens into a shorter focal length one, which when used with the normal position of the lens equals an extension, which is why they're normally used on lenses with a limited close focus range, not view cameras with extendable bellows. If you need that situation with a view camera, just switch to a shorter lens---it will have the same effect without the extra random glass in the way.