Why worry -- other than the fact that it's flammable. All that's needed to start a fire is having the thermistor fail or come loose. I'd use a flameproof material if you're going to insulate.
The flash point (ignition temperature) of cork is, apparently, 300 - 320°C1, which is not, as far as I can tell, a temperature that the heatbed reaches, so, in theory, cork should be safe to use as an insulator. In fact temperatures of around 300C are used in the manufacturing process of some cork products2:
For insulation applications, agglomerates of granules of cork, known as black agglomerates, are employed. They are manufactured in a closed autoclave at high temperature (approximately 300uC) and pressure (around 40 kPa) without the use of adhesive
In addtion, according to Why should we use cork?
Does cork burn?
Cork is a slow combustion material. That is to say, yes it burns but very slowly and it doesn't produce flame so it doesn't spread. Also, when burning, the smoke that it releases is not toxic.
However, I am not sure if all cork is equal, or whether the thickness of the cork can affect the safety. To give a definite figure, I was thinking of using 2 mm - 5 mm thick cork sheeting.
Has anyone experienced, or know of, any burning (or scorching) of cork, when used as a backing insulator to a heatbed, in particular, an aluminium PCB MK3 heatbed?