I regularly just fold up a suit in my pannier (not a garment bag style, but that's certainly a reasonable option). Even when I fold it up the night before, I don't think it ends up particularly wrinkled and I work in a conservative law firm environment. Just wear decent quality wool suits. More important, I think is transporting your dress shirt. I get all of my suits "in a box" from the dry cleaner (folded on a cardboard back and then placed in a plastic bag). Costs a bit more, but they come out of the pannier good as new that way.