I've always understood that it's best to let anything contaminated with water or oil best paint dry for a few days before throwing it away. Whether it's a roller cover, pan, disposable brush, rag, or can, it can't leech into ground water if it's a solid when you throw it away.
If you have a large quantity of paint to dispose of, you can pour it over news papers or kitty litter. It dries much faster that way.