I saw a picture in the last edition of RHS Garden magazine and there was a flower bed, in a very good garden, with all the poppy seed heads left on. They looked very strikingly decorative......still green. It is up to you, in your situation, to decide to leave or cut back. Most people cut back, as they get so messy after flowering is over. This situation was an expertly close planted herbaceous border. You may be more open.

You could cut them all down and tidy up the plant, but leave one seed head on to ripen, then you'll have thousands of free seeds for next year. When it's ripe (if it rattles it's ripe) cut it off and store the seed in a paper bag. Annie