You don't HAVE to, but the OG books recommend cutting it back to 5" right about now.

My 'Karl Foerster' has been cut back for three weeks and new green growth is up well above the cut area. If you don't cut yours soon, you won't be able to.

Here's a thought. Take a 2' long section of very fat (8" diameter) bamboo. Bury it in the ground to the first interior membrane. Cut down your Karl, and put the feathered plumes into the bamboo. A nice dried arrangement for the garden.

You don't have to loose your tall beautys-move them!!

Here's a great trick. Tie the grass in a nice bundle, cut it, then place a stake in the ground and
slip the bundle over the stake. TA DA! Grass on the move!!
You can stake last years grass nearby until the new
growth fills in.

This is great for creating winter intrest too!

Hope this idea helps.