A heated towel rail is just like another radiator - just a different shape. It would be prudent to fit a thermostatic radiator valve (TRV) in much the same way as you would put one on a normal radiator. You 'balance' the water supply to the towel rail in exactly the same way as a normal radiator.
It is not only recomended it is mandatory under part L (energy efficiency) of the building regulations to install thermostatic valves to all radiators including towel ones with the exception of the radiator that is located in the same room as the wall mounted room thermostat.