In most examples I've seen of wiring buttons to GPIO inputs, the button is wired so that when closed, a circuit is completed between the pin and ground, producing a low input on the pin. A pull-up resistor is used to default the input value to high. Code on the Pi can then detect low as a button push.
I've been connecting the buttons to +3.3v so the signal is high as this seems to make more sense and keeps the code more logical but there must be a reason that most people favour connecting to ground. What are the advantages?