(i gather you mean leaving your boiler off and switching it on when you need it?). Many years ago, my step dad was told that if we left the boiler on permanently it would reduce bills as there wouldn't be that initial surge of energy needed to heat up cold water. So we all melted for three months due to the boiler and central heating being on permanantly. It turned out to be completely false, our bill went the same way as most of the heat - through the roof. I'd definitely say only turn on your boiler when you need the hot water.