Domains are property/assets. Just as you have normal everyday physical assets, you are going to have intangible assets as well. Your 401K is not exactly tangible, it is a statement that you receive in the mail every month. If you were to die, there is usually a beneficiary listed on your account, so the company knows who to contact in order to distribute the money.
Your domains are no different. After you get a few domains it will probably be a wise thing to make sure that someone else other than yourself can access them if need be. The worst thing that can happen is that you die, no one can access your accounts (or may even know about them for that matter) and then all of your domains drop because renewal fees are not paid. This happens quite frequently; don’t let this happen to you or your domain portfolio.
You will need to make sure the document that you craft for someone else to handle clearly outlines what to do with your domains once you pass away: sell, continue to monetize, hold, etc. Since there are not very many people who know the domain game, you need to make sure what you instruct your beneficiary to do makes the most sense. If your husband or wife does not know anything about domains it may be in his or her best interest to have someone appointed (who knows domains) to liquidate them and distribute the proceeds, or otherwise manage the domains on behalf of the beneficiary.
Where should this document I crafted reside? It should go to a safety deposit box, a fireproof safe, or someplace that it would be hard to get to if someone wanted to steal it. I would not recommend keeping it on your computer as that can be hacked at any given time.