It's better to divide warm season grasses after they've come out of dormancy, and are beginning to show growth. That includes Miscanthus and Pennisetum.

Barb,
Unless you are gardening on cold clay I would be very suprised if you don't already have some new growth showing on your Miscanthus. I think you could probably start with them now. For the Pennisetums you might want to wait a little longer. AS SOON AS you see new growth on the Pennisetums I would get started on them as well.

Two points here ~
- the earlier you can start dividing the less shoot there is to damage.
- divide before new growth is showing and you run the risk of saving dead bits & throwing out the good bits. (been there done that)

A.