That is all superstition. Shadow Arrow procc rate is 50%. Now, as anyone can testify who has made numerous amount of armor, 50% is not a high percent to be successful. I've failed 10 alone with 56% on armor, heck even on 70%.
This factor also applies with Shadow arrow. Shadow arrow is the only nuke skill Warlocks get, whereas Nightmare is the main skill that is half the damage of Shadow arrow. At lvl 69 Shadow Arrow does almost 10k worth of damage but Nightmare is just a bit over 5k (5k+2.5k+1.25k= 8.75k), and unfortunately you will not see these amounts unless you have 3.00 worth of MCRIT DMG, High MCRIT Rate and atleast 20k MATK. >< Other words, you have to crit to kill.
That being said, it's all luck against that 50% procc rate. The 3 things I strongly advice you and all warlocks to invest is Cast Speed, MCRIT and MDMG so when you see the proc of Shadow arrow you'll see your HP fully restored. Cast Speed obviously faster you release the skilll, the faster the cooldown begins, therefore the faster you can recast those 2 skills.
Plague carrier is also targetable which is why I use it on the tanks, that way I need not worry about Dark Parasites, on myself in PvP.
Just look at the proc rate of the Debuffing effect of Dark Parasite...30% and to this day never notice it proc. That's how rare, and my lock is 69 with 69 Threats, Arrow, Nightmare and almost on Carrier and Parasites.