My last litter I left a natural tail and love it ! There are pics on the forum of Neelix with his natural tail...I do not think that the tail is an issue in agility. I spoke with two separate AKC judges nad they did not feel the tail to be a huge issue .....However if the breeder that you are working with does show AKC they may be hesitant (unless the puppy is paid for in full) picking a pup out for you and leaving a natural tail , not everyone "Likes the look" .. IMHO