The voltage in the power grid is likely to vary with time because the
load on the system varies and the power company can't anticipate it. But, I
wonder if they sometimes boost the voltage a bit just to sell more energy. A
few volts can probably mean millions of dollars of increased income.
It is interesting to note that the power companies decided long ago to
make the system a grounded system. That is equivalent to making the entire
earth one pole of the generator. (they run a wire from one pole deep into
the earth). If they had not done that, thousands of lives would have been
saved over the years. If they had not done that, you could stand on the
ground and reach up with an aluminum ladder or a metal pipe or even a damp
kite string and touch the high voltage lines and you would not get an
electrical shock.
They put isolation transformers on pool lights for safety. If they did
away with the grounded system, it would be the same as providing our power
with an isolation transformer....and many lives would be saved.