If your ears are truly ready to stretch, you should have no problem putting in a taper all at once (unless you're skipping a size or something). When you wait long enough between sizes, your ears will loosen up on their own and stretch up to the next size with no pain and very little resistance.
If you use a taper over the course of a few days, the back of the fistula is going to shrink around the back of the taper and go through more trauma being stretched than the front. That could lead to blowouts, tears, etc very easily.