If you really need to use a capacitor, put a small resistor (say 100 ohm) between the capacitor and the pushbutton to avoid high peak currents.
Another drawback of using capacitors: When the power supply is switched off, the capacitor discharges through the port pin and might damage the micro. You can put an additional resistor in series here, say 560 ohm.
I think the capacitor, if needed, should not exceed 100 nF.

Alfredo del Rio.