If Chrome and Safari are both based on WebKit, does that mean I don't have to test both browsers?
Everything else beside the basic HTML rendering engine is still different. If only basic HTML/CSS compatibility is important, I guess it's fine to test one browser.
However, the JavaScript engines of Chrome and Safari are completely different, vector graphics are rendered by different engines, HTML5/video tag stuff is implemented completely different, etc. The list goes on.
Overall I'd recommend to test with both browsers.
You should test both browsers to be sure. You'll be ok 99% of the time but you want to make sure.