Dwight, I don’t think you should avoid downward stretching. That would be a mistake, IMO. The tunica consists of different surfaces. The strongest, in terms of the density of longitudinal collagen fibers, are the dorsal thickening, which runs lengthwise in the area of the dorsal vein, and the ventral thickenings, which runs on both sides of the corpus spongiosum. There are essentially no longitudinal fibers under the corpus spongiosum.
Your objective in lengthening your tunica should be to work all of these structures. The ventral thickenings can be stretched fairly intensely by pulling your penis up and to the sides (one side, then the other). The dorsal thickening merges with the suspensory ligament, however, and is best stretched by pulling down.
You preferably want to stretch at 4 (or more) different angles: up and to the right, up and to the left, down and to the right, and down and to the left. Straight down using an A-stretch works very well, too, so I would add that to make 5.
Use a “divide and conquer” approach. Stretching down is just as important as stretching up.