In this specific book, about 70% of the paragraphs are separated by an empty line, so it would make sense to define p with a margin. This is a very long book though, and I'm not looking forward the tedious work of applying another class for the other 30% of the paragraphs so they display without this empty line (I'm not lazy, it is just a waste of time when it's not really necessary).
I have no pickle with the <p> <\p>, my only worry is that it won't display properly on some readers. If that's a non-issue, I just leave it as it is.