A question for people who are in petroleum industry. I heard that they add something to the gasoline in the winter when it gets very cold like below 0F so that it is easier to start the car. But it also downgrade the gas, says 91 octane to 90 octane or so. I wonder if that myth is true?

There is a ton of info on this if you google it. Quick answer is, YES, there are definitely differences between summer and winter blends. It doesn't just stop there though; there are regional recipes throughout the country for fuel sold in those areas.

In my area, I believe they are required to make it 10% ethanol for winter.

Major differences for diesel fuel, less so for unleaded. It all has to do with cold starts during the winter.

So does it make sense to store a lower grade gas during the warm season and use it in the cold season? For example, store 89 octane when it is not cold to use it in the cold winter instead of filling up with 91 octane gas.
This is just a thought, not for practical use.

If it has ethanol, it'll go bad unless you use a stabilizer. Even then, is it even worth it to go through that much trouble?