Keep it subtle on the master buss for most types of music....though I'm sure some stuff might enjoy getting hit a little harder.
I've been running an Overstayer Stereo VCA on my master bus on the last bunch of mixes I've done, and it took a few passes to find where it was helping the mix, but I like it now.
Does the FET III have a Mix option, so that you can blend the dry signal with the compressed?
I have the on the Overstayer, and I find that knob to be the most rewarding. First dialing in a certain amount of compression with the Mix knob full wet, and then when I find the flavor I'm after, I turn back the Mix knob and blend the the dry signal back in. Usually end up with it around 2/3 wet.
Post back how you like the FET III. That's a nice chunk of $$$ for one of those!