It's best to test on a Mac rather than relying on the Windows versions, as there are sometimes (albeit relatively minor) differences. If you don't check on a Mac, you'll be taking chances, that's for sure. That said, I don't see too many differences generally—certainly not major ones, anyway. (If you are into pixel perfection, you will get quite a few surprises though. Personally I don't care about that.)
Second hand Macs are pretty cheap and fine for testing purposes (though make sure it's an Intel version).
I use a Mac myself, so this isn't an issue for me (I run Windows on the Mac too, which is handy. It's a pity it doesn't work the other way around.)