Since this is my first year growing it, I can't answer the flower question. When I buy it at the store and have some that will go bad, I snip it in a plastic cup with scissors. I then put about 1 TBS in each ice cube square and fill with water. Once froze, pop them into a baggie. To use (I use in fajitas) just drop a cube into whatever you're making. I use this method with all leftover herbs, but will probably attempt to dry some this fall.

If you let cilantro flower and go to seed, the seeds are the spice known as coriander and will give you a whole different (but good) flavor. Plus you will have more cilantro if you plant the seeds or even just let them self-sow.