In a typical WordPress install we would use a starting template such as underscores which utilizes current best practices and saves time from reinventing the wheel. Underscores is not meant to be used as a Child Theme but rather a starting point for creating a completely custom theme for a project.

With WooCommerce, what is the current best practice for custom theme development? This refers to a custom designed site theme which would not be resold?

Storefront appears to be the official starter theme and is also based off of underscores. All documentation points to not customizing it but instead suggests plugins for minor customizations or Child Theme for major customizations.

If one plans on creating a completely custom look based on the particular projects designs, would it be best to edit the Storefront template directly, knowing that any future relevant updates would have be manually applied on a case by case basis where/if appropriate, or create a Child Theme with customizations allowing the Storefront core code to be updated in the future?