Not every wall needs to be covered floor to ceiling with decor, it's all about creating a sense of balance in the space, so it is definitely ok to leave some walls blank.
Negative, empty space can act as a design statement in itself, and help an overall design feel more calm and harmonious – there are no rules to say every wall should be decorated!
When deciding on wall decor, consider the other design elements in the room, and ask yourself if a blank wall is needed to act as pause where the eye can rest, or, if it needs to be filled to enhance the overall design of the room – sometimes it takes time and a little experimentation to get it right.