Turning off IC's when not used in battery powered circuit
What would be the most efficient method of turning off the 595s when they are not used?

You will have to disable their outputs, this is done by making the not(OE) (output enable) pin HIGH.

Disconnecting the supply lines of the chip will work ONLY if you can guarantee that none of the other pins of the 595 will be high. Because if only one pin is high, it will supply power to the chip. See this EEVBlog video for an explanation on that.