As others ahve said, unless you are one of the "heid bummers" at the wedding it is best to go with the tweed for daytime. In Scotland, though, you will find that many guests will arrive in PC's, daytime or evening, so you could quite reasonably join in. I would leave the fly plaid at home though. Sometimes the groom and best man will wear them to distinguish theselves a little from the herd but others should not for that very reason.