Unless a student receives at least two 18s or 19s, it's better to use standard norms, because they had a much larger standardization sample, and consequently more robust psychometrics. Extended norms are only recommended when a student's scores are high enough (in the ceiling of the standard norms) that it might be of value to try to see where they stand among other students scoring in the ceiling. The rule of thumb for when that might be considered to be the case is when there are at least two 18s or 19s. (And actually, 18s are under consideration mainly for the sake of older students (adolescents) on the WISC, who can't miss as many points before they drop out of the max scaled score; the ceiling of the test is lower for them. For younger students, it really should be 19s.)
