There is no -20 hp in 5E.I wish you were right, but I don't think you are. I think saving your healing spells until someone goes down is absolutely the correct strategy under the 5e rules as written. When a player gets reduced to -20 hit points and you Healing Word them for 4, you end up healing then for 24. How is that not the correct plan?
That's a pretty good idea! Extremely simple but effective.
It's bad action economy because healing a downed PC eats the actions of both characters, while healing a standing PC eats the action of one character (the healer). Action economy is far and away the most important element of 5E combat and giving up actions because you are willing to get dropped is bonkers.