Kerry,
Don't worry about this too much. Putting a lot of thought into heading tags is probably more important to your own development of copy than it is to Google--once you get past the H1 tag, and even that is hardly a noticeable factor. Any well constructed copy is going to have the most important words and concepts at the beginning of the copy and your heading tags just reinforce that.
However, you don't necessarily want to think of lower tier headings as places to put "additional" "keywords". Think of them, maybe, as opportunities to use vocabulary that supports and broadens the concepts of your page's primary terms. I know the web is chock full of landing pages that organize their keywords that way but for the most part, I think that's fruitless for better search results and BORING for the reader.
These subheadings are your opportunity to distinguish your publication from the hoards of others when your audience first skims through your article. Think them through and use them to enhance the reader/user experience first and search engines somewhere after that.