Common GND with Buck Modules?
This is okay, except that your 240 vac negative terminal is NOT ground. The wiring diagram is a little ambiguous there, but I think you understand. 

Best in this case would be a large bread board or something similar to give you a nice working surface, as well as plenty of access to your 5v rail. 

Ideally, you want you ground path (current return path) to be reasonaby near your power source or power path.