Since Ash Gray is highly scripted to mirror the anime, saving at these points is recommended:

Elias tried to walk toward Viridian City, but every time he stepped into the tall grass, no wild Pokémon appeared. Instead, a text box triggered: "Do you remember the storm?"

To avoid losing your journey through the Kanto anime events:

Mark sighed. He reached for the keyboard. He knew what he had to do. Keeping the save file was like keeping a jar of air from a place you’d never visit again. It was stagnation.