It doesn't matter in terms of fusion results but it may make a difference in other ways. The doctor may prefer to use one side over the other depending on your position during the operation. If one side is harder to reach than the other then easy access makes the decision.
It may make a difference to you later. Sometimes patients have pain at the donor graft site. The pain can last months to years. If you tend to sleep on one side more than the other, then you may want to choose the side you don't sleep on for the graft site. Lying on a painful hip can make sleeping even more difficult.