Ive done them without any sealer on mine and not had a problem at all, to be honest i cant imagine why they would need sealer when they are under compression, but i guess if you dont clean the surfaces you might need it, and its certainly not going to cause any harm if you do use it.

Takes about an hour to do the job, but half that time is cleaning the surfaces.

Use a cometic gasket, well worth the extra few quid.