In past years the prices have followed a seasonal price curve and were more predictable, so it was easier to make this decision. The last few years the prices have been so volitile, there is no basis for even a guestimate of the prices this winter. Talk to the buyer at your oil company. I have gotten good advice from mine in the past regarding this.
I would suggest paying all advance payments with a credit card to give yourself extra protection should the oil company go out if business before making good on their commitment to you.