The "feed before flowering" advice can mean simply that the plant becomes unpalatable to rabbits later in the season, but it can also indicate a plant that takes nitrates (or nitrites - never can remember which) from the soil and concentrates them in the foliage. In some plants, especially those grown on heavily fertilized soil, the concentrations can reach dangerous levels for some livestock if eaten in quantity. When feeding a good variety of plant species, it may not be a problem as the rabbits can make their own choices from the greens fed. Many people feel that the rabbits are able to make good choices based on their needs.
Sometimes these directives have been passed down through generations of rabbit raisers by word of mouth. The reasons sometimes get lost along the way. Another one is "never feed mangels before Christmas". As I understand it, it is a question of sugar in the roots changing to starch during storage. But even now I'm not sure of the reasoning. Michaels4gardens has an old-time book with a title something like "Feeding Rabbits and Chickens on Scraps". He may know the answers to some of these questions.
Edited to add: Just thought of something that needs saying. The plant Daucus carota (Queen Anne's lace, wild carrot) is safe, except for the seeds. The seeds have contraceptive properties and should not be fed. I wanted to mention this because late in the season, after flowering, it becomes a concern. You can feed all other parts of the plant right through the season.
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