Dave Liquorice wrote:

In a word yes. *B-) *Not overly important for small loads but heavy ones
like heaters it is better to switch off on the appliance before switching
off at the wall. If nothing else it saves damage to the sockets switching
contacts by the arc that you see.

I suppose that would be the case if the switch in the heater were more
durable than the one in the socket. On the other hand a 13A socket is
fairly cheap and easy to replace but sourcing a replacement switch for a
heater could be more difficult, or even impossible for some Chinese
imports.

--
Mike Clarke