I cull regularly. If I don't like it, it becomes compost or gets traded. If I get too many of one kind, they get traded.

If you have a dahlia that is not vigorous, I would cull it. Put it on the compost pile, and don't give away. Most likely, the tubers from it will not be vigorous, either, and you don't want that to carry on. I cull anything that doesn't grow the way it should, or shows sign of any disease. Mildew, however, is not an internal disease and they shouldn't be culled because of this, if that's the only thing wrong with them (response to an earlier post).

Teresa