WHY do I have to ask?

[McMurdo]

You should not have to ask, I never did ask myself and I wear my kilt everyday. Sometimes it is better to ask forgiveness rather than permission. It says right in your employee handbook re dress code:
"Dresses or skirts should be of appropriate length."
There is nothing inappropriate in the length of a kilt, my only concern would be the tolerance of your workplace, I am lucky in that everyone has accepted my kilt wearing and they have even come to enjoy and look forward to it, however you know your workplace and the people that inhabit it will they accept you dressed as you wish? That is the question you should ask yourself before striding into the office kilted.
One other thing that I should say, if you are going to do it then make sure that your outfit is well put together, and looks as professional as it can be. I would also caution you as well do not go regimental as this could be a problem, also I would be wary of carrying a sgian dubh at work, I have done it, but again you have to gage these concerns yourself.

[James MacMillan]

Sometimes it is better to ask forgiveness rather than permission.

I think that's in the Marine Corps Manual somewhere? Anyway - That's what I would do!