So what you're saying is that I should always keep pins 0 and 1 open ?
Would the same apply to pins 14 - 21, which are also marked for "communication", or can I use those for the LCD ?
Virtually all of the I/O pins on the Atmel chips serve dual purposes. They are available for general purpose I/O (such as driving an LCD display) if they are not being used for one of those other purposes.
Pins 0 and 1 are used for downloading your programs via the bootloader and for the Serial Monitor. If your program does not use the Serial Monitor then you can use it for other purposes after your sketch has been loaded. This is usually more trouble than it is worth unless you are really in need of I/O pins.
On the Mega pins 14 - 19 are used for the additional serial ports available on that device and pins 20 and 21 are used for external interrupts. If you are not using those functions then those pins are available for your LCD.
Don