So, you've been getting this "Master Warning" message for nearly a year and you've ignored it completely, despite the recommendation in the owner's manual.
Is that correct?
Do you think they put the "Master Warning" on the car just to annoy you on cold mornings? I seriously doubt it.
I'm not an expert on the Prius, but there MUST be a reason the warning light comes on when the temperature is at or below freezing. What that reason is I have no idea, but perhaps someone at the Toyota dealer might know.
I suggest, considering the enormous complexity of the Prius drive train, that you stop ignoring the importance of the "Master Warning" light.
I'm willing to bet $50 right now that you will completely ignore my suggestion and go through the coming winter cursing the "Master Warning" light every day as you start and restart your Prius.
Until the day it doesn't start, no matter how many times you try.
Then the "Master Warning" light will get your full attention.