Are your radiators not as hot as they used to be? Cooler at the top than at the bottom? Then there is a chance they may need bleeding, but dependant on what system you have the technique varies.
Open Vented Systems
Turn off at programmer (so demand for the heating and hot water is off).
Position a rag under the valve to catch any escaping water.
Using a radiator key slowly open the radiator bleed valve by turning anti-clockwise.
If air is present it will start to hiss, turn clockwise to close as soon as water is present .
Wipe off any excess water with rag.
Repeat with all affected radiators.
Sealed System :
This one is best done with two people. One to fill the system and check on the pressure gauge and the other to bleed the radiators
Same procedure as above but you will need to repressurise the system i.e. put the water back in.
Locate the filling loop and slowly open -keeping your eye on the pressure gauge.
Once all the radiators are full make sure the system is back up to pressure ( usually 1 to 1.5 mbar, dependant on system).
Always have an old rag or similar item to catch and wipe off any water spilt.
Never take the radiator bleed valve pin completely out.
If you constantly have to bleed the radiators then there is a problem. It is dragging in air from somewhere and will have to be investigated further.
A radiator cooler at the top, (as in the diagram) but hot at the bottom, is an indication that there is air present.