as long as you just want to have a manual control to over-ride the arduino, you can put a switch in the low voltage line between the arduino and the SSR. this will let you swtich between "off" and "computer controled"...
this will not be a 'safety feature' though, and will not positively prevent current from going to the element (like if the SSR failed closed). it would only prevent the arduino from turning the element on. you could also put a switch on the 240v wires, which would positively cut power to the element for safety. or yes, you could do both.
if you just want to prevent or interrupt the arduino from triggering the heater, its best practice to switch the low voltage signal wire instead of the more dangerous 240v. in general, the fewer high voltage wires and switches you have to run, the fewer chances of an accident.