You can use one common wire for all 8 possibles buttons per Shift Register i/c or one common for each switch.

The first save some wires and pins, but result less flexible, with the second you can change a button without need disconnect he other seven.

Depends on space available for Shift Register board, the fist case is more suitable for use inside joystick grip, due space restrictions, e.g. like is done Cougar/Warthog Shift Register.