Depending on the layout of the room, there are many less than favorable areas to position a radiator. The main thing to consider is the distribution of heat, and whether surrounding fixtures and fittings will prevent it from travelling freely around the room.
In smaller spaces, it might be unavoidable to fit a radiator nearby to large furniture, but try to leave as much space as possible between your radiator and a sofa, for example. Our blog on where to position a radiator offers further guidance.