No calculation. It's just a common rating for resistors, and it's never a problem. You have to consider how much power will pass through the component. Consider on a zobel that basically no current will pass through the capacitor that it is teamed up with at frequencies lower than the capacitor cut off. Then above that point, the inductor in front of the zobel will be cutting down power. And how long of duration do you get more than 10 watts going to the speaker? Very short duration.
If you're concerned you can go bigger, or parallel two resistors together of double the R value.
Resistors are rarely put in places with high power demands. That would just make the speaker less effecient.