My wife needs advice...please.

[The Wizard of BC]

My personal advice would be to ask the exact same question but remove four little words from your original question.

If you had reworded you question like this, would the answer be easier, or would the answer be more self evident?

"My wife is making a skirt. She prefers a shorter length than hostess length, but worries that a shorter length would not be proper. She would like to know if any other length would be appropriate for daytime events?"

I find that a lot of people seem to feel that because the fabric is Tartan that there is some special requirement envolved.
Actually you would look at Tartan fabric just as you would any other skirt fabric. Some skirt fabrics lend themselves to one style of skirt over another due to how well they drape. Others, like any plaid, lend themselves to pleats.

Then you take into account the event and the intended use. A skirt made for wear to the office or your job could be different than one made to wear during an evening out dancing or one made to wear going to the grocery store.