You should switch off your central heating before attempting to bleed central heating radiators, to make sure the radiator itself is not too hot to touch.
There is also the possibility that water can spray out of central heating radiators when they are being bled, so it is much safer to turn off the central heating to reduce the risk of scalding hot water emerging.
For further information about safely bleeding central heating radiators, visit our dedicated blog, How To Bleed A Radiator. Or, for inspiration regarding how central heating radiator designs can enhance your home, take a look at our piece entitled, Top 40 Central Heating Radiator Transformations.