Googler Paul Lewis has built a handy CSS tool...
Instead of spending time performing your own tests, you can now use Paul Lewis' CSS Triggers to understand the ramifications of setting the value of a CSS property to something other than the default, or of updating that value.
The CSS Triggers site lists CSS properties alphabetically, and lets you know whether changing their values will trigger layout, print or composite work. Clicking on a property brings up detailed information about what will happen and what the costs might be.
At the moment this only works for Chrome. Lewis is keen to expand the functionality, but at present not many browsers have tools for exposing information about what's going on inside. He is inviting vendors or anyone else who can help to file a Pull Request.
You can read about how Paul made the site in his blog post, here.