Browse website developed in BootCamp via OS X/Safari
You won't be able to browse the website on the Windows BootCamp - as Windows wont' be running while Mac OS X is. You possibly could browse to the bootcamp file system via safari, but the webserver wouldn't be running, so that may not be valid testing if your site is anything more than static files.

You could as you suggest used Parallels/Virtual Box or VMware Fusion to run the Bootcamp Windows instance while the Mac is running - one issue with this though is that if you switch between running Windows as a VM (under one of those products within Mac OS X) and between it running natively via bootcamp (i.e. Mac OS is shutdown, but windows is running) then you may have to keep reactivating windows, as it considers the hardware to be different when you boot it under the alternative method.

Whether or not testing from a Linux machine is important or not, would be up to you. Linux browsers would be a pretty low statistic for most websites. You could easily run Firefox in both Windows and Mac to do your testing - personally, I'd feel also doing it under Linux might be overkill.