Once you brew the "standard" of a certain style, then you can start to tweak and change a recipe, or even just make up your own.
Dunno about that. Brew whatcha like. Who set's the "standard?"
A solid, proven recipe is a great start. But there are any number of solid, proven recipes for a given style and you'll find variation among them.
As far as MO for a base malt, in my experience it doesn't come through in a dominant way in a dark beer. I think it should be fine to use, even if it's not the traditional approach (this is homebrewing, after all).
The only reason I care much about style guidelines is for competition, and that's not so much.
It's also nice to have a touch-stone for other people when they're trying your beer (OK, there's two reasons).