If you are going to let the node go into sleep between sensor readings (f.ex. Wakeup every 5 minutes or so) you don't need the troubles of switching to 1Mhz.
When you sleep the atmega, you also turn off the oscillator for the MCU. So it doesn't use any power.
In fact one could argue that it will result in a longer wake time if you are running at 1Mhz instead of 8Mhz, due to the slower execution speed. And so it could end up using the same amount of power, as when running on 8mhz.
This is however untested/unverified, and just a theory that I have.. :)