There are differing opinions about this in the States. I do wear a kilt to most formal fund-raisers, banquets, etc. and don't feel the need to get "clearance" in advance. I would give special consideration when it comes to a wedding, though. Others on the forum have made the point that there would be potential that a kilt could upstage the bride...not a good outcome.

There are so many factors involved, the tastes of the couple, the sensibilities of their families, the setting for the event... it's hard to have a generic answer. Best bet if you're in doubt: ask the couple and make sure they understand that you won't be offended if they ask you to wear some other attire.

If they embrace the idea, then go for it, but do make an attempt to match the level of formality of your kilt kit with that of the other guests. In other words, if it's a relatively casual, afternoon affair, then a formal PC or doublet based outfit would stick out like a sore thumb. No worries, though, the kilt is as flexible a man's garment as there is. With the right jacket and sporran, a kilt can fit right in to any level of formal or casual setting. For a great view of this flexibility, visit the thread "1 Kilt 10 Looks: A Visual Guide to Formality in Highland Attire" in the General Kilt Talk Forum.

If you could give some more specifics about your particular situation, I'm sure you'll get some great suggestions here.