Amyk is correct. To do this correctly you have to evaluate both the parts involved and the purpose you will put them to.
If you want some general rules of thumb I would give you the following:
1. If your goal is just to get the processor running you will be generally safe with crystals with 10-25% of the nominal design (more on the low end, less on the high. You may be successful with much larger departures but the odds go down the further you go. Again, data sheets are your friend.
2. If you are doing PWM and other similar processes with your processor things will generally work with small variations (again 10-25%) from the nominal value. But be prepared for odd cases that don't work, and behavior that is not optimal.
3. If you are doing clocks and timers you really do have to be guided by the needs of your application. While some of these applications may be OK with a few percent error, others will demand errors that are a tiny fraction of a percent.