This really depends on your situation. I had a lot of additional documents, some were necessary (police certificate for a country which wasn't listed under "required"), others were me playing it safe (a lot of explanation letters - e.g. about why I don't need a PC for a certain country, why we are not providing proof of income for my husband etc.). I also used the comments a lot.
I think if you have a very easy, simple case, then not much else is needed. But as soon as there is just the tiniest possibility of doubt, I think it might be best to play it safe and give them more information about it, so if they have questions, they don't need to send an RFE. That's my take on it.