Sealing pressure treated deck, to be stained later on
I would be concerned that sealing the deck now might create a major problem when it comes time to stain it -- I'm thinking big blotches of different shades caused by differing amounts of stain absorption as a result of the sealing.  Whereas greasy food morsels will at worst be a minor problem that will likely be completely solved by the good cleaning you're going to give the deck before staining anyway.