Always, but then I'm a rider (and horse rider) myself, so of course I would.
Drivers who don't slow down for horses are morons. They forget that horses have just as much right to be on the roads, and in fact were using many of the roads in this country for hundreds of years, before cars and other vehicles were invented.
And before anyone says anything about road tax, there isn't really such a thing, as ALL taxpayers (whether they drive or not) contribute to the roads. Furthermore, with regards to insurance, any responsible horse rider has liability insurance if they ride on the roads.