Soaking the plants for 10 minutes in fresh water will make sure they are fully hydrated so that they don't suck up that bleach water. One cup of bleach in 5 gallons of water is probably not strong enough to do much. A 10% solution for 10 minutes is generally considered safe and has been recommended by more than a few knowledgeable plant people on the board. I use a 10%, 10 minute dip myself and have dipped many orchids with no ill effects. The only things that have ever responded badly to this has been mosses.