YES! Definitely use the green coriander seeds. I wait to pick them until they're plump and round and turn a light green color. They have a lovely, sweet and citrusy taste that you miss out on if you pick them when they're tiny and the petals have just fallen off. In our garden (zone 5), they plump up about a week or two after the petals drop from the flowers.
My husband and I recently made a batch of dill pickles using fresh coriander seeds (we found this recipe: http://www.acouplecooks.com/2012/06/q...) and it was delicious!
You can also wait until they brown up to use them; the citrus flavor will be less intense but still great.