I took one at 9 weeks with my first, then waited until 17 weeks to do them weekly. You could see no bump until 20 weeks for me with my first. I might start a little earlier this time if I notice anything.
I say do what you want, I have a friend who took a photo every day of her pregnancy, stood in the same place in the same pose with generally the same outfit (balck yoga pants, white tank top) and then at the end they did a video of them rapidly changing.....it was really awesome.