Ive been doing it for years. If you get an extension lead, take the earth off that and then plug your power pack into that. By rule of thumb, you shouldnt be able to do any damage to your laptop as this is protected through the neutral wire and the breakers in your house. the earth wire is connected to the case of electrical equipment so if something live inside the machine touches the case, it wont go live, thus saving you from an electric shock. Since a laptop runs on 12v DC usually, then your really not putting yourself our your equipment in imiediate danger. Probably the most unsafe part in doing this is your actuall transformer in your power pack. However you do so at your own risk ;)