... well at least when they're working. But seriously, this question does come up quite a lot. And it's not that surprising because I have heard some horror stories, not least because my industry has its share of idiots and chancers like most others. Jeans and a T shirt should be a definite no no!
Firstly, you're the employer and as such don't feel awkward about asking the question or making (reasonable) stipulations.
Secondly, it's a wedding. Anybody working there should show respect to the people and the occasion. Whether they're a wedding photographer from Reading or a DJ from Ibiza, common sense must prevail.
Now, there is a train of thought in the wedding photography business that us 'Togs' (as for some strange reason we have named ourselves) believe we must be comfortable. This is not unreasonable, and very sensible. But this shouldn't detract from being smart and presentable.
Far be it for me to say what wedding photographers should wear (we're generally big enough and ugly enough to make our own decisions), but I see nothing wrong with wearing a suit (for the men) just like the other male guests. Female photographers, I believe, should be equally smart but obviously not too "dressy", they need to remain practical.
Some photographers will wear a top with a logo. You may or may not find this acceptable, and your view may be effected by the colouring and size of any logo on display.
So, do ask. You may be glad you did!