i would say depends on the baby. some friends have used wipes from day one, i usually leave it till cotton wool is not really enough to do the job anymore! (usually when solids start) but depends on whether your child has sensitive skin
When i was in hospital after having ds i noticed that alot of the 2nd/3rd time mums all had secret supplies of sensitive wipes hidden in their bags for when the midwives weren't looking. Is that not normal or was I just surrounded by naughty mums?
I've been using them from the start - cotton wool and water just don't cut the mustard (iykwim!) with poo! I am using the "sensitive" ones though and we've had no problems with soreness so far (2.5 weeks in!)