To clarify further. Those buttons for reset skin guid and for disable cache only work if CombineCss is true. Our csshandler.ashx serves up the css when that is true and there we have control over cache, the skin guid is appended to the url of csshandler so when it changes all users will load the new css since browser cache is based on url.
If you set CombineCss to false then csshandler is not used and links to all the static css files are included in the head of the page. We have no programmatic control over cache for static css files, they are served directly by IIS.
It is ok to set MinifyCss to false when working on a design but I recommend never set CombineCss to false.
Hope that helps,