I've cleaned tanks, sealed them with stuff like POR-15 etc and find that a little cheap insurance is always good to have. Carry a spare fuel filter and a screwdriver for the unexpected, at 10pm, when the engine goes sput sput. I even put a shutoff valve coming out of my tank in case I have to EVER change it. Tiny, to add to your idea of saving left overs in a tight sealed contain without much air. A trick I learned years ago is to blow into the container which replaces oxygen with your carbon dioxide and quickly put the lid on it. I've done this for years and it works on everything from house paint to urethanes in preserving them.
