Your statement is correct.
You will find that the CSS files within the UNI template are actually set to inherit or point to the base css files. The documentation recommends leaving the inheritance in place, and adding specific styling for only the items you need to modify, ie:
yourtemplate/css/common.css
@import url(../../base/css/common.css);
body {
font-family:Verdana,Arial;
font-size:11px;
color:#333333;
margin:0px;
background-image: url("../images/mainbg.png");
background-repeat: repeat-y;
background-position: center top;
background-color: #666666;
}
In the example above, I am inheriting all values from the base common.css, and only overwriting the body styling with my own.
In my personal experience, I had run into issues with doing it this way, especially when allowing the css files to cache. I have since resorted to removing the inheritance and completely writing my template's stylesheets out (my situation may be very unique though).