[Quoth]

Many actual ereaders only support two levels of indent on the System TOC
Things like image captions in eBooks should use CSS styles in a p tag.
Actual heading levels should really only be used in ebooks when you want a page break or really need it in the TOC. All other Heading-like text should simply be an appropriate CSS style on a p tag. Not all paragraph headings should be in a TOC anyway, only main ones.