To add to what people have said already, you can also get non-rinse shampoos which I love. At work, we use a non-rinse lavender shampoo like this one: Requisite Lavender Shampoo - Coat & Skin Care - Horse Care - Stable & Yard, Equestrian Equipment Robinsons UK
on the horses who work hard in the summer, as it has menthol in it to cool and promote circulation, and it makes them lovely and soft. Another reason not to put leave in conditioner in the mane is that it can make your reins really slippery, not fun!
In regards to manes and tales, like Kayty, we pull manes and trim bridlepaths, and bang the tail above the fetlocks. We don't pull tails, but they get plaited if we're going hunting or to a competition.
Feathers also get trimmed in the summer, beards are trimmed all year round and any of our hairy cobs who might struggle with mud fever have their lower legs clipped for ease of drying and treatment.