you're right in suspecting that testing sites in safari on windows isn't enough. Macs render fonts differently (arguably more accurately) and these subtle differences can affect some designs. Browsershots is a good site, but if you are serious about web design (in that you wish to do it professionally) then it might be worth checking out eBay for an old mac. A g4 capable of running tiger OS can be picked up for around £50.
I have safari for windows but is that really enough?
thanks
This can be pretty good, http://browsershots.org/ to see how the browser interprets the page. Hope that helps.