coordinate system - When to enable 'On the fly' CRS transformation in QGIS? When to turn it off?

I understand what 'On the fly' does, and I have re-read https://docs.qgis.org/2.6/en/docs/training_manual/vector_analysis/reproject_transform.html

I tend to have it turned on all the time, but I don't know if that is correct or not? I am aware it can cause issues.

I have had a look through the related questions and they all tend to be when users are having problems, and further complicated by people asking about their particular datasets and respective CRS's.

Is there simple rule-of-thumb answer to when to have 'On the fly' turned on and when should it be turned off?

Answer

The rule is simple: the on-the-fly reprojection is useful only when you are dealing with layers which have a different CRS and only when your purpose is visualizing multiple layers at once.

Instead, great care is needed when you want to perform spatial operations or when you are visualizing their results because, in general, the rendering of the layers doesn't match to the real results.

In conclusion, it is only thought for the displaying of layers which have different source CRS. You may always have it enabled, but I think that the unique rule-of-thumb is remembering it is activated.

I generally prefer to work with layers which have the same CRS and, if this doesn't happen, I reproject the layers of interest to the same CRS: following this philosophy, the on-the-fly reprojection is generally disabled. I enable it only when I need to quickly visualising some layers or when dealing with WMS services because they are generally provided using Geographic Coordinate Systems.