I don't roll mine. I have it plainly stated on my invoice that they expire at the end of the month. Keep in mind that if you roll them you are limiting yourself for the next month. For example:
Your client retains you for 10 hours a week but rolls 20 over. Next month they get 60 hours. That's 20 you don't have for a new client.
It may not seem like much now, but if you plan to work 40 hours a week, you would need 4 clients at 10 hours each week. If each client rolls 10 hours one month you've just added an entire week of work.
Anyway -- no, I don't roll mine over.