Well, this is actually specified indirectly in the accuracy numbers. For instance, 0.8% accuracy on the 999.9µA range hints you not to trust any reading under 8.2µA (0.8%+3), because the instrument can display any random number there. In these instances, some manufactures choose to display "0", others just trust you that you won't take seriously those very low readings.
On top of that, as a general rule, as you know, no measurement at the bottom of any given range (say under 5%) should be considered very accurate.