If I remove the terminal blocks from the Nano board (ATmeg328P) so I have only holes in the board, can I use un-used holes in the board for soldering components? It would be kind of like a perf-board for prototyping? Are there voltages on the Analog or Digital pins that would affect my circuit even if the Analog or Digital pins are not use in the software? The components I would be soldering to the board would be push buttons, resistors and a power on-off switch to a 9VDC battery. Are the Analog and Digital pins disconnected and open circuit if not used in the program?
First of all, don't run 9V to any of the pins except VIN.
Yes, Vin is the only pin 9VDC is on.
Unused pins are not like unconnected pins. You can not apply voltages beyond a certain range, and they have capacitance that could affect some unspecified circuit that you wish to attach in this way. I think it would be good if you gave some details of what you are going to attach to the unused processor pins. However, it might be helpful to mention that for the most part, the pins are in a high impedance state by default at power up, except for some that the Arduino initialize at power up.
As mentioned, under some circumstances you may get away with using an ‘unused’ pin as a place to connect miscellaneous components. However, in principle, it is unwise and you should try to find another solution. Using pins 0, 1 or 13 may give strange effects possibly preventing the uploading of programs. Use of any other pin could also cause damage if the wrong sketch is loaded in error. Special care is necessary if your use of any pin brings it electrically ‘close’ to one of the power rails.
Okay, thank you all for your input. I will come up with a different solution that doesn't connect to un-used pins.
I've done some "dead bug" construction. You can often make connections in mid air if the other ends of the components are connected to something rigid (like an active Arduino pin pad).