Which html div classes are relevant when styling Views in Drupal 7 (for CSS)?
I can only speak to my personal experience. I use these classes to style my views primarily:


.view-myview .view-content the main content of the view, encapsulates all rows
.view-myview .views-row this is the row in your view, add a border, some margins, etc.
.view-myview .views-field-myfield a singular field in a view


You also get views-row-even, views-row-odd and views-row-first, views-row-last, etc. 

The classes can handle all tags, it doesn't matter if they're divs, spans, etc. I did a presentation at DrupalCon SF 2009 which explains a lot of different ways to customize views if you're interested it is here: http://sf2010.drupal.org/conference/sessions/jquery-views-and-view-themeing.html