I know you should never approach a horse from behind. Horses are skittish animals and any movement they can't see, especially from behind is something they perceive as a threat and they'll lash out.
Around a strange horse, I absolutely agree not to approach from the rear unless you must (e.g. haltering in a standing stall).
But my old racing-bred T'bred was absolutely bombproof. We loaned her to the Special Olympics, and she was good as gold. Around the barn she'd have three kids painting her hooves with Hoofflex, two brushing her sides, one brushing her tail, two braiding her mane, and two feeding her peppermints (all under alert adult supervision of course, and NO Running In The Barn). Never so much as a startle - she was a good, good girl. RIP Full of Grace "Gracie".