I tend to use 33pF also and again with no problem. I dare say you could remove the capacitors alltogether and it may work without a problem - but make 1000 units and see how many work! As I said previously, the actual value isn't super critical - at this range of capacitance just adding an ic socket adds a few pF, the pcb adds a few pF, the mounting of the pcb adds a few pF ,if your hand is near the micro - a few pF there. As you can see, the actual capacitance can vary quite a bit depending on a particular construction, temperature humidity...... so again, in this application the precise capacitance isn't too critical - the crystal does a very good job of oscillating at the required frequency or near enough so that we don't have a problem. If it was a crytal oscillator for a frequency counter- then we might want to be a bit more careful and maybe add an oven for the crystal to keep it at a constant temperature and have a trimmer capacitor to tweak the frequency. It all comes down to what accuracy you require - in most micro circuits we don't require too much.