For the past month or so, I've been trying out Guard + LiveReload on a Rails project. So far I've found it to help tremendously with Pixel Perfect when matching the look specified in PSDs. It is quite seamless - to enable it amounts to running `guard` in the command line inside the project rails directory and clicking on the LR icon in the corner of the Firefox window.
Follow the installation instructions for these gems:
Guard
Guard::LiveReload
If you're like me and prefer to use Firebug, grab the Firefox extension instead of installing the LiveReload Safari/Chrome extension.
Read: Instantaneous CSS Updates Directly From Source Files with LiveReload