Do dishes used for preparing food need to be washed with soap?
Human saliva is not your only concern.  There are a variety of pathogens that could be problematic if food preparation items (hands, utensils, cutting boards, counter tops, and containers of all sorts) are not properly sanitized. This includes potential pathogens that already exist on the raw food itself. Soap is not necessarily required, but some type of sanitizer is.  Dish soap just happens to also remove grease, and is most commonly found in the home kitchen.