As long as the cables reach, I don't imagine it matters much where you put it. If you can, take some tape and scrap wood or something approx the size of the gauge you are looking at and go spend some time trying places out. If you find a spot that looks promising, tape the dummy gauge there, get on your bike, and check the visibility. It's really going to depend on your bike and your personal preferences. One last thing; make sure that you can either find or fabricate a mount so that the actual gauge will work in your preferred spot.
If you are really a hardware hacker, maybe you could even integrate one of those electronic all in one gauges with the back of a riding glove, then run the cable up your sleeve and have a connector near your waist that attaches to the bike...