Discretion to treat disputed tax liabilities as non-default while appeal is pending, while undisputed taxes must be recovered. An assessing officer may, in his discretion, treat an assessee as not being in default as to amounts that are specifically disputed while an appeal remains pending; that discretion applies only to disputed taxes. Assessing officers must nonetheless actively recover undisputed taxes and must not allow assessees to withhold payment of undisputed demands because an appeal has been filed, with senior officers supervising compliance.