Having been caught out recently when a web site I launched displayed perfectly on IE, Firefox, Chrome and Safari on Windows but was corrupted when viewed using Safari on the Mac
If it's a major concern to start doing a lot of testing on a Mac, then I would definitely suggest buying a second hand Mac, or perhaps building a Hackintosh. The former gets you up and running quickly, the latter gives you a lot of power for the same price.
For just the odd piece of testing, running OS X in VMWare on your current PC is a cheaper option.