Steven: I think you as the TD can switch from 10 handed to 9 handed anytime you feel that it is better for the event.
Some things to consider, are you going to need any additional dealers, can you send a dealer home, where in the event structure are you, do you have any "small, stud type tables" that are, at best, uncomfortable for 10 players and many other factors that need to be considered.
I don't know of any "rules" that govern such an action.
Chet