It is usually, though not always, the case that where to place commas when writing can be guided by how the sentence would be spoken aloud. If you make a slight pause or change in inflection, it is a good place to use a comma.

Now, in the above sentence I wondered if I should put a comma either before or after 'that'. I have known those who would put one in both cases. I just wonder how they would speak the sentence. I, myself, would possibly use a comma after 'that' (It is usually, though not always, the case that, where to place commas when writing...).

It certainly can be a delicate matter of context and personal style, although basic rules always apply.

(Yes, it is also true that the use of "I, myself,.." can be debated. No doubt there are opinions.)