Theming Panels with Zen Grids
Your answer is quite flexible actually. 

You select the one column template and disable drupal standard regions and then just add a php filter custom content for each row in your grid. You add the grid classes inside your php filter and then use block render or views_embed_view. 

Another more clean solution would be to create your own layout plugins for panels. It is not as difficult as it sounds. Just download and fork panels_extra_layout and you have everything you need in order to create your own custom and reusable grids. 

Have fun.