My mum would always (and still does say sometimes) that babies need water and must get so thirsty, but this isn't true.. babies don't get as thirsty as we do, plus the formula is mainly water.. so they are getting all they need from it. The only time I gave Bethanie water before she was eating food properly was when: A)she wouldn't go long between feeds, then hardly drink anything when she did, B)when she was constipated (a little prune juice or fresh squeezed orange in the water also helps).
Obviously, sometimes if it's hot or you just feel baby needs a drink.. go with what you think. Try to stick to only a few ounces of water too, as too much can cause more harm than good.