Signal Buffers

When exactly are signal buffer's necessary? Say i'm daisy chaining 30 shift registers together, will I need a buffer at certain points in the chain or will decoupling capacitors be enough to ensure signal integrity?

You will have 30 Serial clocks and latchpins in parallel from the arduino. May want to buffer those after 15 devices. At the same time, the arduino's +/-20mA drive capability should be plenty for 74HC595 inputs.

Ser out to Ser in will be buffered by the chips themselves. Definitely add 0.1uF/100nF cap across +5/Gnd at each chip.