As long as it's well-done, either is fine.
I did read one ebook, poorly edited if at all, in which the author switched back and forth -- and this was not a matter of style or experimental writing.
One of my biggest complaints about self-published ebooks is the lack of editing. One book had several misspelled words per paragraph, glaring grammatical errors throughout, andquotes and dialogue so poorly used that I had to guess as to who was saying what.