the only rule with fashion is there are no rules, but your work has to look like other similarly done campaigns.
In other words, original, to a point, but not so far out as to be off the wall.
As a few have mentioned, its commercial photography, for a client and trying to get paying people to buy stuff.
So whatever the client wants works, and most fashion shoots have a story board and a look, so you match the look and the storyboard and the art director (AD) is happy and the client is happy and folks buy the stuff you shot and you get paid.
I can't think that any clients ASK about fstops..its just not done, and most wouldn't even know the term. You shoot to capture the look the AD wants and thats all you do.
It's not rocket science, fashion is big money shooting with a very commercial end result in mind..get ladies (90% of the fashion sutomers are ladies) with money to like the mood your images evoke and want to buy the product you shot because they envision themselves in the story you shot.
Every single fashion campaign and photo shoot revolves around that concept.
