When I had the horses I always was very cautious where ever I rode and only cantered where I had a clear view ahead. Never heard stopping was bad for them - you certainly do learn something new every day don't you! Obviously where I went wrong...
Mind you, changing the subject slightly, a cyclist ran into the back of my friends car a few weeks ago, she'd stopped on a red light at a pedestrian crossing because young children were crossing, oh and a red light showed.... The cyclist had a real go at her about why she'd stopped. He did apologise later when he realised what a load of whatsit he spouted
Obviously the whole Highway code needs to be reviewed