Hebridean sheep are increasingly used by top sheepdog trainers because they move readily, yet still tend to remain together as a flock.
However, Hebs need to get used to a dog in stages without any undue stress or pressure. Once they know what to expect, these intelligent animals will respond accordingly.
In contrast, "undogged" sheep may panic on the first few occassions - especially when approached by an untrained dog. These inexperienced sheep may flee, scale "insurmountable" fences and also potentially injure themselves.
This obviously isn't good for the sheep, but it also may teach a dog (and the sheep) to have bad habits as well as shred the handler's nerve!
If you are a novice sheepdog handler wishing to use Hebs to train your dog, then it is advisable to discuss with the seller whether the sheep are suitable for this purpose (i.e depending on your own requirements specifically ask whether the sheep have been used for sheepdog training or have been worked by an experienced dog).
See here for a personal account from Martin Andrews about using Hebrideans to train working dogs.