It is not necessary. It is helpful if the horse does not have an acre of space. I show the horse the clippers. I let the horse sniff the clippers. I do not train the horse to target the clippers.
Clippers can be dangerous for the horse to lip or bite.
Most horses will sniff the clippers. I then pat the horse with the clippers in my hand. This will cause the cord to flop about since the clippers are not plugged in. If your horse is already dancing this is what you need to work on. Click and reward for any signs of calmness.