Couple of things going on here...
First off, the laws of inheritance and the convenience of procedures to implement wills vary significantly by jurisdiction. US states differ markedly, and by your avatar, I imagine that the law of no US state is applicable to you. The second layer on top of this is the uneven recognition of domain names as property that would be subject to a will in the first place. The third complication is what, if any, procedures your registrar (likely in another jurisdiction) would or would not have for recognizing, say, letters of estate administration from some other jurisdiction.
On a very general level, it doesn't hurt to have something in your will for whatever that may be worth, but as you point out, having a prompt means to effectively access and control the relevant registrar account(s) is of more practical importance.
Depending on your circumstances, it can be simpler to administer your domain names through a business entity such as a corporation or LLC, and to have provisions for assuming control of the company. Unlike people, companies don't die unexpected or inconvenient deaths, and so there is no "dead registrant" problem requiring an authorized company officer from assuming control of the relevant accounts.
There was a situation with which a number of registrars are familiar from a few years back in which a gentleman was apparently registering vast numbers of domain names and PPC accounts in his mother's name as what appeared to be a tax dodge. Upon her death, various siblings of his found out about the domain names, and a tedious and time consuming dispute arose between him and his siblings over control of the domain names, with the various registrars and PPC providers wanting nothing more than to not be in the middle of a dispute as bitter as only an intra-family one can be. Consequently, various service providers updated their terms of service, which is likely the first place you should look for clues about how they may react to receipt of various official documents on the occasion of your demise.