Hmm, I used a parafin heater in my last greenhouse, and never ever trimmed the wick and it seemed to just carry on regardless. :-\ I didn't notice it using more or less fuel, especially considering if it was going to be really really cold I turned the wick out more for more heat so the fuel useage varied anyhows..... I got so fed up with parrafin as I seemed to be buying it weekly and if it ran out in the middle of a very cold night, well, all that hard work keeping the house heated was wasted! I guess my spend back then - about 3 or 4 years ago, was about a fiver a week.