I never could get them to stay attached to the rocks for long that way. Maybe you might have better luck. I just place mine against the tank and hold it in place with one of those cleaning magnets. As long as it's fresh and you keep the bag sealed up, I wouldn't worry about boiling or rinsing.
However I also feed with Romaine lettuce and store it in the freezer. Whenever I feed, I thaw it out in very hot water which softens it. Then I rinse it off in DI water before I put it in the tank.