I think it really depends on the horse and the person. I give from the hand to all the horses I have now, but I have had horses you just couldnt give treats to. I know professional trainers who treat and some who are totally against it. If you have their respect I see no problem giving treats.
I will give a treat about once a week, sometimes twice, when I go out and catch my horse. But that's it. I never give it after riding, or when I'm in the barn because they often start looking for it. With my horses that are stalled constantly, they don't get 'em. Oh, and I give them by hand.
I give by hand too, unless I happen to find a loose treat, and I just pop it into her feed bucket. I do tell her to take it nice (not like she understands) and when i'm done and she looks for it, I say "you ate them all piggy"
I remember when I was out in the pasture with Blu for the first time. It was getting dark but we were playing with his extra feel barrel. (half the pasture is a garden in the spring and summer) I found some leftover, late watermelons. I broke one open and it was juicy and ripe. Blu and I bashed melons for an hour. They were delicious and I wasn't afraid to just share a chunk with my baby.
Lol. He would bite where I bit. I doubt he noticed but it's funny. Lol.
I really like giving treats at totally random times just because I am out visiting or I feel like it, but I don't often remember, or think about it...so our horses don't get them very often. :(
When we have apples and carrots, I will take them out and hand feed or trow them in their buckets with their dinner depending on my mood. I don't make a big deal out of treats, and so it is not a big deal to the horses. I have no issues with hand feeding my horses treats.
One of our newest members, ak_showgirl, often brings big bags of carrots out, which are given as treats. If it weren't for her, our horses would get way fewer treats. :)