From my experience you're OK like 90% of the time. Sometimes fonts look a little different, etc, but generally you're OK.

I agree with the grandparent though, you need to worry more about Safari because of the iPhone users that hit your site. Case in point I recently saw a site work perfectly on in all major browsers on the desktop, but Safari would get caught in a loop of repainting. The result was a webpage that looked like it hung the browser, but it was actually taking like a minute to finish rendering the page.