I don't really have much say in this matter but the 2 times that Dobe has actually been bathed with shampoo, I did condition his mane and tail. It was so soft and wasn't nearly as stiff and fragile feeling afterward. I can only imagine what his would look like if I kept mane/tail bagged and washed/conditioned. Wow. I know that when I wash my hair, I can't stand not to have conditioner in it because it just feels dry and brittle if I don't condition.
I never shampoo my horses, but I will spot shampoo for manure stains, etc. I rinse weekly in the summer, and not at all in the winter. Rinsing lifts the dirt and sweat off, but leaves the natural oils, which makes for healthier skin and hair. I do mix either Pantene Pro V conditioner or Infusium in a spray bottle at a ratio of 3-to-1 with water, and spray the mane and tail each time before grooming, allowing it to set and dry. I finger-comb manes and tails, and once a week I use a vent brush to be thorough. I never brush/comb manes or tails wet (increases breakage). Both of my horses look like they have mane and tail extensions, which they do not. If money is tight, I will use Suave conditioner with coconut, and cut it at the same ratio, but I get the best results with Pantene, and it is still by far cheaper then Mane-n-Tail, or Cowboy Magic. There is no build-up, since the horses are rinsed regularily.