I would say that if you do not have a Dom then at that moment in time you're not really a submissive; a prospective submissive yes, submissive orientation yes (I appreciate this is probably a highly debateable argument). If you have a Dom then you should wear the clothes your Dom likes or as he instructs. If he has no preference or you don't have a Dom then you are free to wear what you please. Submissiveness is inside your body not outside!

Personally I like a sub to be dressed according to mood and situation. Maybe I want to set the mood and will give dress instructions, or leave it up to the sub to express herself through her clothing.