Not all magnetic stir bars survive such harsh conditions, most of the cheap ones will demagnetize at temperatures as low as 180*C. Also not everyone have such powerful magnetic hot-plates that are capable to reach over 500*C, otherwise you will never reach inside temperature close to even 300*C.
I forgot to mention only about boiling chips from some crushed white ceramics that I used in the second stage.
For the first stage it's not necessary at all, moreover boiling will be only within first hour or less, later there will be just evaporation. (my setup was 3L beaker and 1.5kW cheap-as-shit hotplate with thermal relay)