If your cilantro is already flowering - it is on its way to being coriander. The usefulness of the leaves has pretty well past. Staggering the plantings of seed will probably work for you but you may need to wait until late in the season when the weather cools off. My best cilantro plants are the ones that survive the winter and show up growing nicely in the spring.
Dill has a tendency to not flower all at once. I suppose that you could remove the flowers and force it to produce more flowers at a later time. I'm not sure if that would be best, however.
If you were thinking of using the leaves, once again and like cilantro, that moment is probably past.
For use of the flowers as a flavoring for dill pickles - you may want to cut the dill and dry it. If you do not have cucumbers ready for pickling, the dry dill will be useful for that later in the season. I'm quite sure that it would freeze well, also. For drying, make sure it is out of direct sunlight, tie stems into small bunches and hang them upside down where there's some air movement. Easy.
Steve