I am running a rate of around 95% each week or every other week from IAH-PHL-IAH.....usually Thursday or Friday late flight from IAH and 545 am on Monday morning from PHL.
To me, CPU is just a marketing tool to fill seats when there are empty ones up front to make those who really arent entitled to it cause not paying the BF ticket, feel special and continue to fly to keep status to get more CPU's. Does not really cost the airline anything I dont think except the cost of that "awesome" meal that is provided.
So with that, if you book times in which flights are usually full up front your chances of CPU clearing are smaller.