I would go under dressed. But I wouldn't be embarrassed either way if people want to make you feel uncomfortable just because of what you are wearing I would choose not to go to a party with those kid of people
Over dressed. Because you can always dress down once arriving at a function but its very hard to dress up. To be under dressed when the expectations are higher can cause a lot more social grief than turning up overdressed to something more casual, because events that require formal dress have higher social stakes (weddings, balls, cocktail parties, formal functions). Informal events usually mean people are already in the groove and will not be hassled if you turn up overdressed but quietly slip to the loo to take off the high heels, un tuck your shirt and remove your tie, put a friends jacket on or borrow a t-shirt....
That's an interesting question. I think overdressed would be better, though even that could be embarrassing if you were really super overdressed (like wore cocktail to what turned out to be smart casual. Dress codes can be really confusing for women too. For men the same suit would work for occasions that require subtly different degrees of dressiness for women. That kind of thing bewilders me, but it doesn't come up often for me anyway.
That's a great question. I'd opt for under dressed as for me, that would likely still be smart casual. I'd be very unlikely to turn up to a party in trackie pants and a hoodie, so my typical smart casual could likely still lend itself to a party theme. However, if I was overdressed and went full party frock and heels to a casual attire party, I would feel pretty embarrassed and it's not a look you can easily shrug off as casual.
This question reminds me of a friend's first date in the 'sixties. Boyfriend asked her to a party on his parent's "boat". She turned up in jeans and sneakers to a very formal do on a very big yacht. She still married him.
Overdressed any time. I would be mortified to be underdressed. Much rather put too much effort into an outfit than to appear not to care by being underdressed.
This happened to us last weekend at a wedding. There was no dress code on the invitation. Fortunately my husband could ditch his jacket and my outfit wasn't embarrassingly over the top.