ergophobe - 5:47 pm on Mar 7, 2013 (gmt 0)
>>background images don't load up until the document is sure which image is required.
I think that's really your answer for the specific question.
On the general concept...
Yes, your CSS base theme files will be part of the theme cascade as will your template files, your theme functions and so forth.
The Drupal template cascade is complex because you can target very specifically and through many levels. Essentially, for a template, Drupal starts with the most specific possibility and keeps working up towards the system level until it finds something. It loads the first file that meets the requirement and ignores the others higher in the hierarchy.
CSS/JS is a little different in that it works it's way up constructing an array of all the files it will load and running any aggregation routines.
If you add a subtheme.css file, it will not conflict with the base theme files and therefore everything will get loaded. That may be what you want. In that case, you can still override specific rules - your subtheme customizations should be the last CSS file loaded, so the normal CSS cascade applies.
If, however, the base theme CSS has more cruft than you want, you could also create a file of the same name, load it in your subtheme and and thereby block loading of the base theme file of the same name.
This file could be empty, or it could take a subset of rules from the base theme file. This will reduce the size of your total CSS load, but it will also make it harder to integrate any theme updates if you want to be able to do that.
But to answer your question, I would say it's best to have as little as needed in your subtheme, but no less. In other words, if something in the base theme is really getting in your way, then yes, remove it and load something else. If it's just a handful of style rules that are the problem, though, just override them with the normal CSS cascade.
I'm afraid that may have made even less sense than my original answer... :-(