One bit of trivia from my experience using the NOOK Simple Touch:
When an epub uses <p> the NOOK overrides my css to format the parargraph in one respect, that is, if I use the css to set the paragraph indent to zero, the NOOK still indents paragraphs. Clearly the device has some internal settings that trump the css; I don't know how to change those.
I got around this by changing all the <p> tags to something else, like <standard>. When I set the properties for <standard> in the css, then the NOOK obediently omits the indentations.