Total access needed or is it more like spying?

Is total full disclosure the best thing? If your SO has social network accounts, (Fluther would be one), IM accounts, email and phone should your SO have unfettered access to check it out whenever they feel without asking? I can see that it would assure not one is not cheating on the other (or just better at hiding it). Since it can’t guarantee against cheating would it seem like unnecessary spying? Should there be enough trust where you don’t have to think about who or what your SO is doing with they accounts? Or, is there an in between?