Personally I try and hose my horse down or at least sponge the sweat off the horse every time I ride. I can't imagine that it's comfortable for a horse to have the sweaty hair stuck together on it's back and girth area.
I think as long as you are sure to scrape off the water you put on and continue to hose/scrape until there is no heat it is fine to do so any time you ride. The main problem with hosing a sweaty horse is if you don't do this the water you put on gets hot and makes the horse hotter than if you had left them alone. Make sense? So, cool them out first or be very sure to cool them thoroughly.