Having only had a justice of the peace wedding myself (24 years
ago) I may not be the best to answer this one. However considering
how many times you will put on the gown before the wedding. What
with trying it on, alterations, showing it off to your wedding
party so they can coordinate etc. I would consider it already
having been worn before the wedding. Now if you mean wear it out on
the town or something like that. While I don't think it will mean
the wedding would be cancelled but it does seem a tiny bit
disrespectful to me. But I will be the first to admit to being a
bit old fashioned on that score.