Im with Airhead on that one.
Grease or oil on the gaskets, primarily to try and avoid them sticking, so they might be reused again, is good practice.
And an easy way of avoiding frustrating and time consuming scraping off of old gasket!
In saying that tho, there's little chance of a thin smear of liquid gasket doing any harm if its used for "belts and braces" sealing.
Or even do both (liquid gasket one side, grease on the other) for frequently removed casings, so that the gasket stays stuck to one side only, therefore avoiding it being torn when the casing is removed.
But really if the faces are in good order then its not really necessary.
I'd avoid the butter or jam tho!
And Im not going anywhere near the suggestion of bodily fluids!!
V.