This really depends on what the item will be dressed with or used for. Bed side storage needs to be at a similar height to the top of the mattress. TV units ideally want to be at a comfortable viewing height, so when you are sat watching TV, the centre of the screen is at eye height so you are not stretching your neck looking too much up or down. For sideboards, consider if you will have decorative items placed on top or if a pendant light will be hanging over or a picture hung behind etc.