I have a bright-blue file folder labelled "IMPORTANT DOCUMENTS" that holds my passport, social security card, birth certificate, and the like. I have a handful of physical files---a box's worth---and so I keep it in there. Maybe not the safest option, if someone is looking for it, but safe deposit boxes have always struck me as overly paranoid. How often do people break into your house? Do people break into your house to steal your identity, or to steal your (stupid and non-Mustachain) giant flat-screen TV?
I have everything scanned and saved on my computer as well.
If these are your only physical documents, why not just keep them in a large mailing envelope (or a file folder inside such an envelope)? That way, you can put them in a safe/hidden/wherever place, and you don't need to worry about stuff falling out, like you might with just a file folder. And you can scrawl on it with a sharpie, "TAX RECORDS (DANGER! VERY BORING! NO NEED TO OPEN!)".
There are plenty of places in your house that you can stick a 9x13 mailing envelope and no one will find it.