If you ask this question to a lawyer the answer is obvious – of course they do…
Verbal agreements are impossible to enforce if the goodwill that was there when they were established no longer exists. Getting agreements in writing is a good idea. However the cost associated with this can be prohibitive if legal documents are used.
A letter between the parties with the signatures notarized may be enough. If the issues are simple (i.e. a certain asset is to be owned by one party or there is a change in parenting time) and you are able to clearly state the agreement then use a letter. I might be tempted to say that an emails confirming your agreement is ok but if there is something of value at steak then get a piece of paper with a notarized signature. One party could claim that an email was not received or that the reply was faked and while paper is old school, if the signatures are notarized it will be difficult for one party to claim that they did not sign it.
I am not suggesting that you can use a letter for everything and you should consult your lawyer as to when one might be appropriate.
Just use your common sense on this one.