Our family 'rule of thumb' is never to upstage the groom - if he is wearing a PC as in this pic, I will wear an Argyle. I have been to a 'formal' wedding, where PC's were stipulated. On this occasion, the Groom and bestman wore Fly plaids with theirs, the guests did not.
I have seen many weddings, with many variations of this theme, and other weddings which were 'casual' ones with jacobite shirts. So as someone said earlier, don't stress, just go and enjoy.
PS, to the 'tie police' I know I should be wearing a plain tie, I have enough of them - I just can't stop myself wearing my heart on my sleeve - or in this case on my neck. So apologies before anyone points this out to me...