I have 20 shift registers daisy chained across 2 of these boards to drive the display in this clip.
20 SPI.transfer()s are used to send the data out.
I generally see no reason to commit a pin to MR, a specific project might need that.
OE can be tied low, or connected to a PWM capable pin for LED dimming as one option.
Where seperate SS lines comes in handier is for addressing parts which contain multiple registers, such as MAX7219, so you are not forced to send the data thru numerous parts just to update 1 register in one chip.
