My opinion is that the bacteria on a few decorations is a small fraction, and you will see no effect from killing them. If you want sparkly clean decorations, bleach is good. I rinse heavily, soak in a high dose of DeChlor (or any other Na thiosulfate dechlorinator, but obviously don't waste your money on water conditioners). If you use plastic plants, they're good to go once they're dry. Better to let rocks air dry, preferably in the sun, for a few days. We did it a lot when I was at a LFS, because customers want shiny clean tanks. Probably not worth the effort at home, where most people want a more natural look anyway.