Always aim to provide the load capacitance specified on the crystal's datasheet. I use the formula on page 2

here, which is pretty standard.

Turning the formula around, and assuming the two load capacitors are equal gives:

C = 2(C

_{L} - C

_{S})

Where C is the load

*capacitor* value, C

_{L} is the load

*capacitance* specified in the crystal's datasheet, and C

_{S} is stray capacitance.

When I do the maths, assuming C

_{L} of 18pF and C

_{S} of 5pF, I get 26pF, so I use 27pF (standard value) caps with 18pF crystals.

Many crystals will run without load capacitors, and that might be OK on a breadboard in a pinch, but anything else is just Bad Design. When something isn't working, I don't know why I'd want to have to worry about whether the oscillator is running correctly. I can create more than enough of my own problems in other areas, so I figure I might as well get the oscillator right and not have to add that to the list of possible causes