thanks for your question and the suggestion.
First thing: You should generally not edit the plugin's CSS files directly, as gixxer188 is suggesting, as all of those modifications would be lost after an update. Instead, all changes should go into the "Custom CSS" textarea on the "Plugin Options" screen.
Regarding your suggestion: I can understand that such a feature does have use cases, however, I feel that this does not affect a lot of people and would therefore not really be worth the extra trouble for them.
In the same manner, you could take a look at the CSS selectors like
td:last, which are available in CSS3 or (for backwards-compatibility) in jQuery.
Additionally, this might be achievable with a "WP-Table Reloaded Extension", where you could hook into a few plugin filters and add those classes yourself, after doing the checks/calculations in PHP.