if you're a week into flowering, then it's still possible. it will stress the plant a little bit, but if you're topping because you're worried the plant will get too tall or stretch too much, then that's a good idea. letting the plant get too tall and burn against your light or stretch past your light (remember, they double to triple in size when they flower!) is probably worse than topping it right now.
if you think the plant won't have the danger of getting too tall or whatever, then don't top it since the main branch produces your largest cola!