I was scrounging around in the HTML5 spec on the W3C site looking for something else and found an answer to a question I had asked about earlier on Web Teacher. In the section titled Annotations for assistive technology products (ARIA) I found the answer to my question about aria-level.
A table in the spec detailing the HTML5 language feature with its “strong native semantics and implied ARIA semantics” shows the following for h1 through h6 elements.
Native Semantic Element: h1-h6 element that does not have an hgroup ancestor
Implied ARIA semantics: heading role, with the aria-level property set to the element’s outline depth
So the answer is do include a value for aria-level in headings that are not nested in an hgroup. If the heading is part of an hgroup, include the aria-level attribute in the hgroup element.
The value associated with the aria-level would be dependent on the element’s position in the document outline. Here’s an example with an hgroup:
<hgroup role="heading" aria-level="2">
Without an hgroup:
<h1 role="heading" aria-level="2">
The entire table at W3C is very useful. I urge you to read the whole section on Annotations for assistive technology products (ARIA).