I feed my snails some veggies once a week. Drop in some celery or broccoli or lettuce or spinach. These are all things I have seen my snail eat. The rest of the time they will clean up the tank.
Either way will probably work fine. I know I generally just drop it in fresh and the snails have no problem getting to it on the surface. They are always on it by the next morning anyway. But you may not want to look at a piece of floating veggie so a quick blanching in hot water will still be fine.
You could use a veggie clip or stick it partially under a rock or decoration as well. Your snails will probably be able to breed whether you feed them or not. Just keep the feeding to minimum and you will have healthy snails with strong shells.
Your snails will definitely be able to feed even if you specifically try not to feed them, so don't worry about them starving.
If you want more snails, then feeding them will certainly do that. I'd err on the side of caution starting out. Snails are known to go through population explosions when there is a surplus of food, and that can get a little ugly (picture egg clutches on every available surface).