Water changing methods

While we're on the subject of water changes (thanks Karen), I've got
a question I'd like to bounce off y'all. Is it more important to
control the temperature of the new water by balancing the hot and cold
tap mix or to avoid the use of hot water tank water religiously?

The problem with using cold tap water, is that in the winter, the
water supply is very close to the freezing level and a 20% water
change drops the tank temperature by about 10F which is touchy for
the fish. The problem with using partial hot water from a heater
is the possibility of increased levels of dissolved Cu (copper).
I'm less concerned about Fe or levels of hardness from the hot water.
Another alternative would be to keep some rather large containers filled
with cold water and allow them to rise to room temperature overnight.
Since I use a very nifty jet-pump driven water siphon this would mean
using buckets again. I'm loathe to do that because pouring water into
a tank from a pail disturbs the floating plants and stirs up the
bottom of the tank too much with a potential for an resultant algae
bloom. I don't like to disturb the substrate unnecessarily esp. if
there is a layer of mulm or detritus on the surface.

What do you experts do?

Steve             in   (oddly) still sunny Vancouver BC