I would think that it depends on what you are doing -- why you are wearing the kilt for this or that specific occasion. Unless it is to "dress up" "in costume" to represent some specific time in the historical past (as opposed to just wearing your kilt because for you, it is a natural item of dress), then I would think wearing a dirk (or any other such out-of-time or out-of-place thing) to be inappropriate. [And depending on where you live, could constitute "going armed" and require a license]. One of the problems I think many of us confront is that just putting on and going out and about in a kilt is in-itself regarded by way too many as "dress up" or wearing a "costume." Don't get me wrong. There is nothing wrong with wearing costumes; it can be a lot of fun at appropriate times and places. But, if every time someone sees someone else wearing a kilt and dressed-out in what is obviously (other than the kilt itself) an attempt to "portray" some kind of character, whether fictional or historic, it just adds to the notion that being kilted MEANS being in costume. An attitude many of us (well, at least I) wished were not so.