Genuine gaskets will seal perfectly well fitted dry. Greasing them makes them easier to remove when a casing has to be opened for whatever reason. I grease gaskets, just because it saves a lot of hassle and time removing traces of old gasket before fitting new replacement. I agree that re using old gaskets is not a good idea, they are relatively cheap compared to having to do the job again because of a leak! I grease all gaskets except head gaskets.