The approach which I have adopted, is to feed once the fruit 'sets' ie appears as small swellings at the back of the flower head. Feeding prior to this will encourage the whole plant to grow, but perhaps at the expense of making fruit, ie tomatoes, peppers etc. Once the fruit has appeared, the feed should be heavy in potassium, as this feeds the fruit, rather than the plant itself. Standard tomato feeds are formulated this way.
I don't feed beans, just make sure the ground is sufficiently rich to start with, using manure, compost and a top dressing of blood fish and bone.