Assuming you are using pull-up resistors, lower resistance is more immune to noise/interference. I'd try starting with 10K and go-down if you have problems.
The only trade-off is current. i.e. If you pull-down a 10K resistor from 5V, that's 0.5mA. A 1K resistor takes 5mA. With switches/buttons a few mA is not a problem. (Unless maybe if you press all of the buttons at once. 
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