No, you can flower anytime you want, but remeber, less veg. time equals less yeild.

If you're growing from seed, there is a 50/50 chance it will be female, but I strongly believe if you took good care of your plants and the dark period was 100% dark, then your chances of females is better.

If you're growing from a clone though, you know it's a female, so flower whenever you want to.