I have always used a magnetic stirrer, from the beginning of my DIY juice.
I have had mag-stirrers for 50 yrs, so it wasn't even a thought.
And the answer is 'no'......never had to heat it first, even max VG.....that is what
different sized stir bars are for, and a retriever.
It evenly and consistently mixes from top to bottom.
If you wish to stir in that fashion, just get a Badger 121 paint stirrer and be done with it. About $11 on Amazon.