Can I safely toggle the CS pin without sending any data?
The HC595 doesn't have a CS input, it's not an 'SPI' device, though with care it can be driven from an SPI master. Data is always clocked into the shift register with the clock line. The Latch input can be coerced into doing some of the job of a CS line, like transferring the S/R data into the output latches, but it can't be used to disable the S/R input.

There are several ways to drive HC595s from an SPI master.

a) Connect all the devices in a long chain, and shift valid data into all of them. 

b) Have a seperate clock line going to each group of 595s, with shared data and CS, so the data in their S/Rs is not disturbed, so can be latched into the outputs.

c) Or have a seperate CS/latch line going to each 595 group, with shared clock and data, so that although rubbish is shifted into the S/R, it doesn't get latched to the outputs, and when you do want to update them, you shift in valid data.

Depending on your SPI library, this may entail some manual bit banging to separate the groups.