Having gotten most of my boys as youngsters (2-3 yr old) and been around too many mouthy warmbloods, the rule around here was NO hand feeding. They did not need encouragement, and I was (then) always boarding, so didnt want them expecting treats from everyone else too. Once they grow up and show they can handle it, I will hand feed at times, and do carrot stretches and such, but they must be polite.
When boarding, it drove me crazy when one of mine would suddenly get super mouthy and pushy and I'd find out that so and so was feeding them treats. I don't want people feeding my horses anything without permission, no matter what age, but hand feeding these young, dumb warmbloods just created problems, IMO.
My horse doesn't get hand fed. When I first got her, everyone in the barn would hand feed her then she started getting nippy. She has to do lots of dealings with non-horse people so I don't need a nippy horse.
I do it. Its how I've get my horse to come down from the back field when I whistle and call his name (when I'm too lazy to hike the several hundred yards up the hill)
He'll travel far and wide for a carrot!
The few times he's tried to get mouthy/pushy, he gets a quick, sharp elbow in his muzzle and he suddenly remembers his manners. But he is primarily only handled by me - sometimes the BO, who doesn't take any sh*t either. And barring any extreme life circumstances, he will spend the rest of his life with me (or her, per my Bill of Sale)
OTOH, if he were being led and handled by more people on a regular basis, I probably wouldn't feed him by hand, since I would not want to be responsible for him getting too pushy with someone else less experienced and accidentally scaring them or (gfb) hurting them.