I do.
Once left a very quiet 4yr old tacked up, when the phone rang, went in to answer it, took a bit longer than planned and when i went back he was lying down without any plans to get back up again, tack all neat and in place still!! so i sat on him lying down, have a very cute pic of me sat on him, with dog in arms, and horse still fast asleep!!
There arn't many i'd trust like him tho!!
I only do it, if I've tacked up and then realised I've forgotten something and I have to run to get it. I can trust Super in the stable with her tack - she just stands and waits for me to come back.
I'd never leave a horse for more then 5 mins with their tack on.
I leave on temporarily whilst tacking up others, preparing to go etc, but absolutely NEVER leave for even a second unsafely - with reins hanging etc, because it only takes a second for them to catch on something.
i hate to see horses being over worked in riding schools, and when they stand in stables saddled up, it doesn't take long to tack up afterall - give them a break!
i do if i need to grab hat/whip etc but make one of the kids hold her reins away from the bolt. before i went back to the yard we are on a horse caught his headcollar on the bolt ,pulled back and broke his jaw.he had to be put down. hence why i wont leave mine in their stables with a headcollar on either.