Exactly how accurate are 555 timers anyway?
It largely depends on the value and types of the timing resistor and especially the timing capacitor
Comparing a 555 circuit to the built-in RC oscillator of a controller, you have to consider that the latter is part of the same die, so will be more or less temperature stabilized ('though hotter when the controller works harder and vv), while with a 555, the components are prey to temperature changes no matter what the 555 does. Even a light draft can really muck things up and even in a sealed box the temperature changes in the 555 will make some turbulence.
Give a man a fish... Here's the pole.
Experiment! Test it!
That way you'll learn a lot about it - teachings that stick (contrary to what you're just told and forget in a short time).
If you do, try with different types of caps (they account for the major instability and temp-dependence) where the electrolyte is the worst (i.e. really bad) and NP0 caps is the best obviously.
How to measure:
If you have a frequency controller it's easy.
If you don't, program a microcontroller to count eg. 1,000 or 1,000,000 periods (or whatever will take you to say 10 to 100 seconds) and use a stop watch for the measure.
An X-tal controlled micro could be programmed as a timer or low frequency counter and if you've perfect pitch, or a (music) keyboard handy, arrange the frequency to be in the range loosely from 100Hz to 4kHz and compare with the musical notes.
If you want to learn about the 555 in general, get a couple and experiment till they smoke
I'll recommend a thorough read of Tonys tutorial on the 555
and experiment while reading.
When you've been through all the circuits and all the text on that page, you'll know more about the 555 than many electronics engineers! And you'll have fun along the way.
And when you're done, checking out his capacitor tutorial
might not be a bad idea.