HTML Markup – Pages and Page Templates

Hello! I know this is ‘back to basics’, but I just want to be absolutely clear that I’m doing the right thing, going forward.

When does it make sense to include HTML markup directly into a page (with the WordPress text editor), and when does it make sense to add HTML markup in a custom template for that page?

For example, if you’re making use of something like Zurb Foundation’s grid system to structure a page’s unique content (this could obviously be simple or complicated), would you build that grid in the page itself?

A particular concern would be if a client without much or any experience needed to edit the content. As a relative beginner I’m just not sure of the best way to manage this.