I have separate machines, but it's more out of convenience than out of privacy or paranoia. My desktop is running Windows for gaming, and that's also where any of my personal stuff is stored. My laptop is running *nix, and anything infosec related (code snippets, bookmarks, half-written future tutorials for EZ that will never be published, etc.) is kept on that. So no, I wouldn't say I enforce a strict separation, but I find it keeps things a lot neater to have a dedicated machine.
If you're concerned privacy-wise, I'd advise reading
lucid's anonymity tutorial, as it will give you some guidance in VM usage, bootable USBs, clearing your machine's logs, etc.