That's according to the Tenth Circuit this week in United States v. Aragon. In Aragon, the Tenth Circuit reversed the defendant's guideline sentence on grounds that the judge had insufficient evidence to support his drug-quantity estimate. The photos in the case were a poor basis for determining how much weight was attributable to the drugs, and how much to the packaging. The Court noted "the great disparities between gross and net weights in other cases," listing several of those cases.
In the end, yes, the judge may estimate drug quantity. But "the best evidence of net weight is net weight itself." Any other evidence must be viewed with caution.