There are rigid rules as to how to properly wear a kilt as a costume. Follow those for weddings and formal occasions, if you like.

There currently a set of rules being formed for the "proper wear" of casual kilts by people on forums like this one. I think we need to be cautious about becoming a shadow fashion of traditional kilt wearing fashion. I think casual kilt wearing is too closely tied to formal kilt wearing styles.
Utilikilts started a trend away from this with their non-tartan macho style and the trend is growing.

For casual wear, my guideline is a question.
"What would I wear with shorts?"
If kilts are garments and not costumes, anything goes. The kilt look must be allowed to grow into its place in current fashions, not placed in a predetermined place and not allowed to grow. That is the way with all garments. Traditional kilts are garments as much as anything else when worn casually.
If it looks right to you, it is right.

Some people will draw a line on how a kilt is worn. They will say something like, "you can wear a kilt with boots but not sandals and socks," which is total rubbish. Jan Bruyndonckx pulls off the socks with sandals look quite well here.
http://users.telenet.be/jbruyndonckx/kilt.html
Socks and sandals have been worn together throughout history. The Vikings spring to mind. The current fashion is not to wear socks with sandals but the current fashion is also pants, pants, and more pants.
Nobody here seems to mind breaking that fashion. Why perpetuate rules that limit kilt styles?