It depends on what you like.

If you want to collect seeds, leave the seedhead until the stem behind it has turned tan. The seeds will then be ripe. Then give the plant a haircut and save the seeds.

If you like the bushy look, you can leave the flowers until the plant looks ratty, and then give it a haircut.

Festuca self-sows in my zone, so I give some haircuts as soon as the flower starts to fade, but before the seeds ripen. Other plants, in beds where I would like some volunteers, are left alone.

Did you save the seed? Would you be willing to share or trade? Let me know. Thank.