It depends on you project and your workflow.
For a lot of projects, the ./www folder would be sufficient as already mentioned, however there are some other folders that could be good to have depending on what aspects of the cli you are using.
Examples:
./mergesfor platform specific HTML/CSS/JS overrides./.cordovafor cli hooks (like before_build, after_plugin_add, etc)
Plus anything else custom you might want to keep out of ./www during development. For example, I have a ./src folder and the contents are concatenated and added to ./www as part of our build process. Our unit tests are also outside of ./www.
Instead of including a specific folder, I have a .gitignore that keeps build artefacts like ./platforms/* and ./plugins/* out of version control.