I also had the idea that I would buy nursing bras to use in pregnancy as well, but unfortunately the sizing was too different for me.
You might feel in pregnancy that your breasts are HUGE but believe me, they will get even bigger once you start breastfeeding!
The biggest your breasts will get will be in the first month of breastfeeding. They gradually start to get smaller as your breasts get more efficient at producing the right amount of milk. But your band size (underbust measurement) will get rapidly smaller after you give birth, so that when your breasts are the same size as when you were pregnant, your ribcage will be much slimmer.
The only way to use bras for both pregnancy and nursing is if when pregnant you can't fasten them up at the back, but need to use an insert panel (bra extender that you can buy in many department stores).
However, you will need a couple of nursing bras in a bigger cup size for the early weeks when your breasts will be huge and tender - if your bra is too tight during that stage, you won't want to wear it at all.
Oh, and you don't strictly NEED to get the nursing bras with the special flaps. You can always unhook the back of the bra and then lift up the front. I did that in the latter stages of breastfeeding when I was only breastfeeding at home at naptime or bedtime, my breasts were smaller and I didn't want to buy new nursing bras in a smaller size; I just bought non-wired bras that I could use after I stopped breastfeeding as well.