Disabling digital input buffers on output only pins (PIC18)
No, don't disable the digital inputs when using the pin in digital mode.  The digital input circuit doesn't really consume power as long as the pin voltage is solidly high or solidly low.  You don't save anything by turning it off, and it might cause trouble.

All pins that can be analog inputs wake up that way because it is most tolerant of the external circuit.  If a pin is held near the middle of its voltage range, then the digital input circuit in the PIC could draw unnecessary power and possibly even oscillate.  So the PIC starts out in the most tolerant mode and the firmware then switches specific pins to other modes as only it knows how the micro is being used.