Darren of ProBlogger gives some good advice on how to post with the sub-nice idea:

The way I look at is that each Blog should be focused on a niche - but that each post (page) within it should focus in on a sub-niche of the larger one.

Whilst there is a place for the larger post that is a little more general in topic - its can often be more effective to break such posts up into smaller ones (a series if you like) and make each part focus upon one element (keyword).

Your blog will perform better overall, and draw in more readers, if it focuses on one idea. However, within that idea are many different topics - many keywords. Your posts should revolve around as small a component of that larger picture as possible. That way, each post is rich in its own area, but adds also to defining the big picture of what you stand for.