You should really make sure that you always have access to monthly records of your income from interest or dividends.
Why monthly and not just year-end? Well, suppose you move to another state -- even if you don't think you will. If you live in two states during one calendar year, you have to file tax returns for both states.
And guess what? They will want to know how much interest income you earned while living in that state! So if you move in, say, September, you will need to know how much interest income you earned up until September and how much you earned from October through the end of the year. You don't necessarily need to keep paper records, though -- if you receive electronic statements, your financial institution will have those available online.