Use a PC PSU for powering a Raspberry Pi cluster and number of 5V+ cables
Yes, you should use all the 5V conductors and at least as many GND to reduce voltage drop over the cable; using extra grounds would only help.   With 5 supply and 5 return wires, each conductor will be carrying almost 5A.  Even this starts to push the limit of what you would want going over 20AWG.