The proper use of Ripple, to avoid being scammed, is to have Trust to only one gateway, one which you actually trust to redeem the IOUs you receive. That way there is no way for debt to be rippled through your account. Only trust more than one gateway after you are confortable with the system and you know how rippling works.
Not really.
You just have to base your trust on rational thinking rather than greed. Some folks are confused on what trust means.
Hint: You ought not to trust all your funds not even cumulative.
You should definitely only "trust" entities which you actually trust to honor their IOUs, but additionally trusting only one entity per currency is probably a good idea. Trusting more than one increases default risk substantially, because if either defaults, you probably lose all of your money.