In most cases they need to be removed. Most cables rely on the items they feed to show they exist and plastering over removes the identification of cable runs from positions of switches. There are some exceptions but it would require more information than you can really give here and better to get an electrician to visit and advise.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!