For slanting chimneys without (very good) holds on one wall (and which aren't significantly flared), back on the higher wall is, in general, the "correct" posture. The reason is that the difficulty of moving up in chimneys is typically associated with getting your back off the wall. (Scraping your back up the wall, thereby covering it with gobies, is one of the tell-tale signs of the beginning chimneyer). If you are "lying down" in the chimney, then getting your back off the wall (either with just hand pressure or hand and one foot pressure) means that you have to work against both body weight and the wedging force exerted by the leg(s). If you are "standing up" in the chimney, than hand pressure just has to balance the wedging force of the feet in order to free up the back. So in general you won't have to work as hard if you face the "correct" way.

Put more briefly, you want gravity to help, not hinder, shifting your back up.