Update: New findings to correct old findings: http://mathemaglitch.blogspot.com/2012/02/icon-update.html.
In a previous thread on how to generate energy without eating food, I posted that I was collecting data on Icons and that I would make a post when I had enough data to give some real numbers.
I have over 300 icon uses recorded now, so I figured I'd give a little update. I'm level 60 so I have 2860 maximum energy and I pay 1441 currants for a tithe. Over a little more than 300 icon uses I had an average of 709 energy per revere and an average of (exactly) 4 reveres per tithe. Reveres per tithe is definitely random, ranging from 2 to 7 (though 3, 4, and 5 are far more likely).
Based on the assumption that energy return from revering is a percentage of maximum energy, it is pretty safe to say that tithing is more efficient than food as a way of getting energy. I've done a more in depth analysis of this here: http://mathemaglitch.blogspot.com/2012/02/icons.html