For maximum power saving sleep is not sufficient. He must go down to 1 MHz in order to be able to lower the voltage. It saves more power to use the build in RC oscillator instead of a crystal. It also uses less power to a 1 MHz oscillator instead of dividing a higher clock down to 1 MHz. It does NOT save any additional power to lower the clock any further. This is because the power consumption per tick depends only on the current mode. So if you sleep during unused ticks having a lower clock frequency will keep the processor longer awake. Although this should make no difference it will as soon as other peripherals come into play. As a rule of thumb you should stick to the highest possible frequency at the lowest admissible voltage. It does not hurt to increase the voltage slightly but it will of course increase power consumption again.
In addition the RC oscillator allows for the shortest start up time after sleep reducing power to a crystal even further. Unfortunately RC has the poorest frequency stability of all.
In addition you should shut down all peripherals that are not needed before going to sleep, e.g. unneeded counters, AD converter, SPI / serial interface, WD, brownout detector and so on BEFORE you enter sleep mode.