What must be done to hide a grid line when creating callouts for a model?

To effectively hide a grid line when creating callouts for a model in Revit, using the visibility settings is a direct approach. This lets you selectively turn off the grid lines without affecting the overall model data. Furthermore, if there is a need for a more comprehensive change across multiple views or if certain views utilize a specific set of visibility settings, adjusting or applying a view template can also hide the grid lines consistently.

Additionally, while deleting the grid line would remove it permanently from the model, this is often not the desired action as it could impact other parts of the project where the grid line is still needed for reference.

Thus, employing a combination of visibility settings, view templates, or other means to control visibility ensures the grid line can be hidden effectively while maintaining the integrity of the model as a whole. All the mentioned methods provide viable solutions, making "all of these" the most accurate response.