I have been using coolers for a while and preheat with the hottest tap water I have usually around 130 F. I am not cheap but I would rather spend my money on malt and hops. If I can save on propane or natural gas use (by heating my strike water to a lower temp) it my opinion its worth it. Whether you preheat or not you still have to have you system dialed in to be consistent. Just another brewing peccadillo!
just as a comparison on my 10 gallon rubbermaid cooler the difference is 2 degrees....not sure you would notice much saving...you are heating water somewhere either your water heater or the strike water....6 of one half dozen of another...lol