I've heard photographers say take them between 6.5-7 months, so you aren't too big, your nose is not wide and you don't have the discoloration. I need to start making preparations. I just made 24 weeks today! But I guess size plays a role as well. Some women don't show until late.
I did mine at 36 weeks as my bump wasn't big until then :-) I'm now 38 w 5 d and ppl still think I'm due only in september!
I got mine done at 30 weeks I think.. it was right before all my strech marks showed up. Lol
I didn't get them done but if I had I probably would have done them around 30wks because I was big and round by then.
Whenever you want, more than likely when you are showing (6 1/2 + months)