I think letting them out is a good idea, they need to get used to general noises and general goings on around the house like the TV, washing machine etc and get to learn that if they are in one part of the house they have to go find their tray etc.
My friend had a kitten who was scared of stairs for the first few weeks because he had never seen them before - he was kept in a downstairs room.
Before you let them out, blue tack down your valuables
