Washing clothes for an newborn
I always used a mild detergent for newborn clothes. I started out with something like dreft or a free and clear brand of detergent. This always was gentle on their skin. However, with my second and third children I just threw the clothes in with the regular wash and thankfully there were no allergies or rashes... so regular detergent worked well for my children.
I think it is up to the mother's discretion how to wash the clothes. To be safe use a mild detergent like a free & clear or a dreft detergent... but if you do not have a history of detergent sensitivity you could try putting it in with the regular wash as I did.