Ideally everything should be fed from the same 5V supply. That way all 5V and all grounds will be tied up correctly.
I think the arduino board's 5V pin can also be used to power it via an external supply (or you can use its USB) port for this purpose - the 5V being taken from the same supply that's being used to power the other circuits.
Just ensure the external 5V supply is properly regulated and does not exceed 5V else there is risk of damaging the arduino board.