Tons of dead snails in a tank is worse than tons of live ones, in my opinion. They don't vacuum well being heavier... and it would foul up the water really quick. (That is, assuming the tank has gravel.) If it doesn't have gravel, you can just scoop them up, but if this were the case you could scoop the live ones up anyway...
It depends on what kind of snails, too. Some types will congregate on a piece of vegetable, which can then be taken out, others don't.
Some people also say loaches will eat 'em.