I think a lot has to do with the type and style of kilt you have and what you plan to do in your kilt.

If you are wearing a Utilikilt it is acceptable to wear it very low on the hips and have the hem well below the knee. That is how it was designed to be worn.
If you are speaking of a true Contemporary Style kilt you probably will want to wear it a bit higher on the knee. They are designed to be worn more like a traditional kilt.

One caution. If you are hiking or walking any distance I strongly suggest you keep the hem no lower than the middle of the kneecap. All those pleats swishing against the back of your calves has been known to chafe right thru the skin and draw blood!