The problem with putting fish into an uncycled tank is they create a lot of ammonia, and there is little bacteria to eat it up. Once a tank is cycled, you essentially have the amount of good bacteria you need to eat what ammonia is produced.
Thus, once your tank is cycled, you can start adding fish. Just be careful: if you add too many at once, you end up creating more ammonia than there is bacteria to eat it again. So you go slow: a cycled tank will produce more bacteria to handle the load quicker than an uncycled tank.
So, snails don't creat a ton of ammonia, unless you have a ton of snails. Plus, they are pretty tough little buggers. Putting a few snails in will create small, steady supplies of ammonia that will give the bacteria somethin to eat, resulting in a smoother bacteria colony growth. Once your tank is cycled then, you can start adding in your fish.
The only problem with smaller snails is that they tend to reproduce like crazy. You can control it by feeding less, killing/removing them by hand or stocking fish that eat snails.
It all depends on what you want. If you like snails, which do have their useful functions in an aquarium, then get a few from your local store. They tend to consider them pests and will often give them away. I like malaysian trumpet snails (MTS) and ramshorns best. They are neat to watch and eat leftover food.
If you like large snails, a mystery snail might be a good purchase, but I've never kept them, so I don't know how they would handle an uncycled tank.