Well technically, when you jelqing, you use both hands, so by alternating you covert each side.
With jelqing, your trying to force blood in your dick. by doing a grip and pushing the blood to the head and your dick must NOT be at 100% hard, it as to stay between 40% and 80%. If it's a 100% hard, it will be too much pressure and blood vessels could breach. So when you pump the blood, do it by wave, so that why it slow rhythmic movement.
So it does'nt really matter if you dont cover every inch. The fact that it not 100% hard, help stretch the skin too.