I don't like to leave plaster exposed; however, the risk is greatly diminished when the pool is covered with a solid cover. A solid cover will prevent direct sunlight exposure, and it will trap moisture under the cover, which will help reduce drying and cracking.
I would keep it as close to the bottom of the tile as possible. When freezing is going to happen, you should make sure that the level is a few inches below the tile. You should have a submersible pump available to lower the water level as needed.
A lot also depends on how cold it gets where you are. If you get a hard freeze with thick ice, then it's more important to keep the water below the tile line.
How thick does the ice get, and what type of cover do you have?