Unless you want bacteria/germs/dirt to slide in on the knife and get on the flesh of what you eat you wash all melons too, don't you?

No because I don't eat the outside skins of melons. Sometimes eat the rind.  No thinking about that doesn't bother me because I think OK if I eat a little dirt and bacteria - can't be any worse than most of what I already have inside me!  Nothing better on a hot day in midsummer than a slice of watermelon.