Not necessarily. What you can show if you fit into a company culture or stick out like a sore thumb. You need to stick out enough to be memorable, but not over or under do it.
You don't want to dress as casually as they do since they already have gigs, but I wouldn't go overboard. An oxford and casual sport coat or light sweater, dark blue jeans or colored khakis and some nice sneaks or casual shoes (chukkas,...
What type of creative agency? Ad agency? Depends on the agency and how so called the creative they are. I worked for a ton of them in the city and depending on he agency and how corporate they are you can scale the formality up or down....
The crossover has been happening for years. Noah, a brand by current supreme designer Brendan Babenzien was one of the early examples in the early 00's. Nom de Guerre was merging both as well in mid 2000's. It's commonplace at this point.