I am pretty sure it depends on how serious you want to take your showing career. If you are just going to enter RC events ie. youngstock classes then yes - feel free to trim her to within an inch of her life - but if you are going to purely enter traditional cob youngstock classes then I would assume the same rules would apply as showing M&M animals in their relevant breed classes - you really need to seek professional advice from like-minded showing persons.
The Show Ring Forum is a superb place to start - you need to pay a subscription but if you are serious it is a very small fee to pay.