Most freshwater crabs you can find are Fiddler crabs, which need at least some salt in the water to survive for any length of time. I have mine in with african cichlids, with just a little salt (my water is already pretty hard). The most important thing is that they have an easy way to get out of the water, or else they'll die or find a way to climb out of the tank altogether. I think this is the main reason why so many people have trouble with their crabs climbing out of the tank, no matter what they do to try to prevent it.
-J