Usually, I make a Child Style of any style I make/download, and set the doc_margin and doc_width differently to make it fluid (or fixed, if the Style was originally fluid). It's worked fine for me so far, and that way my users can choose their own preference.
Is there, like.... something wrong with doing it that way? Does it consume more resources or something?
Nothing wrong with adding a Fluid child style. But you just have to make sure that it doesn't break your design when expanded or contracted. Thats why most people with fluid styles make use of repeating patterns/images.