On a typical stepper the linearity of the main 1.8deg steps is about 1.5%. Halfstepping is about the same. Once you get into microstepping below that then its down to the driver and the load. A typical 8bit driver will give a further 1 - 1.5% non-linearity on an unloaded motor but the load will determine the accuracy to a great extent as the magnetic forces act as a spring against the load. Microstepping (other than halfstep) should never be used to improve resolution only smoothness of motion.