If the child's style.css contains this at the top
/* Import layout */
then you are working on top of (overriding) the parent's css, just redeclare only what you want changed below that line. So yes, this will contain only changes but technically it loaded the parent's into it already.
This is the most efficient way for most use cases, so stick to this unless you are completely reconstructing the parent's layout or require lots of modifications then just remove that line and do everything from scratch.
Also, at the very top of child's style.css make sure this line
Template: twentyten is always there, it tells WP that this theme is a child of that parent.