This article is from the Water for coffee FAQ, by Jim Schulman with numerous contributions by others.
There are several websites that insist good coffee water is
fresh. Some go so far as to change the water in their tanks every few
hours, lest the water gets too warm from the machine's heat and go
stale. As justification they mention that fresh water has dissolved
gasses, and stale water does not. Most municipal tap water has less
then 5 mg/l dissolved oxygen, since it is foolishly allowed to go
stale in while sitting in purification ponds. Even if you attach an
aquarium aerator to your water intake, the extra dissolved oxygen
won't survive heating to brewing temperature (although really gassed
up water may do some amusing things to your boiler). Needless to say,
this is one of the more silly ideas I came across.
Dissolved chlorine and other gasses are also removed by
standing and heating, so carbon filtering is not strictly necessary.
The exception is when your municipal waterboard does its disinfection
with chloramines. This is a fully soluble form of chlorine that
remains in the water. The good news is that it isn't nearly as
tasteable as regular chlorine. But it does have a taste; and it can
and should be removed by carbon filtering in water for food or drink.