I know this post is a bit old, but I do want to comment.
First, what is the purpose of the blog? Many people blog to document their lives or to just share things with family and friends. When it comes to these blogs, writing anything is fine.
Since you are on an affiliate marketing forum, I'm assuming this blog is going to be monetized or you have plans of monetizing it. In which case, you should definitely
1 - Pick a theme
2 - Stick to the theme
You can deviate every once in a while, and make it clear that this isn't the typical post. One way to do this is by giving that post a different category.
As someone mentioned earlier, visitors will follow a blog based on its theme. They will go back for more if they like what you write. Deviate too much and they may lose interest.
THAT being said, there are SOME blogs that write about plenty of different things, but have a loyal following. For example, I know of a blog that just complains about social issues, political issues, economic issues, or simply everyday life issues, and people follow the blog because they just want to see what this person has to say and what they will complain about next. I suppose the theme could be "complain" but it's definitely not the typical theme.