Is it necessary to have insulator sheets between PCBs and antistatic mats?
A decent quality rubber antistatic mat will have a low end top surface conductivity of \$ 10M \Omega \$. Working with an audio circuit where the highest impedance is a \$100k\Omega\$? You'll never have a problem. Working with a \$10M\Omega\$ pH sensor? Leakage will cause you problems. This is a question that needs to be answered on a project by project basis, based off the circuit and antistatic mat.