I always washed and flat ironed mine about 4-5 days prior to getting a a relaxer(My scalp would burn if I wash any closer to the relaxer date). I would normally get relaxers every 3-4 months so I would have a lottttt of new growth. I did it in order for the stylist to be able to easily get through it. I went to a professional, so she never had a hard time figuring out where my NG ended and etc. I never had any overlap. I think the flat ironed NG still had a slight wave, so it is not like it was too hard to see. Since you're doing this at home(I assume) I wouldn't recommend it though. However, if you do flat iron it anyway, don't get it bone straight. That way you will be able to know what hair is NG...