There is a lot to be said for dollar-cost averaging--- which is what you are talking about doing by putting more money in the depressed sectors--- but in this case can delay thinking about that until you get close to the asset allocation you want to have. If you want 20 percent in commodity related investments but have only 2 percent, you really don't have the portfolio you intend to have.
As I have pointed out a number of times, people were saying energy was "over" several years ago and it hasn't quit yet. Will commodities continue to rise from here? I don't know. What I do know is that those who read a crystal ball and saw that commodity prices were at a peak have been wrong month after month. Someday they will be right. I just don't know when. Neither do you. Neither do they.
That argues for getting to your chosen allocation first, then rebalancing when the proportions are changed.
Scott