Yes is the short answer. Any time you are planing on generating money, you should have a plan. No plan means no real focus. There may be some 1/1000 percent of a chance you will succeed, but I haven't met them yet. If you have already started and have generated an income, record how. Doing so will give you material for use in expanding your business faster.
The web is so large today that it is possible to make money on just about any topic. No matter what you choose to cover, you will find a product and affiliate scheme to complement it. If you choose a topic that interests you, then you will write better content, produce better videos, and have a more engaging website than if you choose a topic of only passing interest. The world is a large and diverse place, so don’t worry that there won’t be enough people interested in your topic. As the saying goes, “if you build it, they will come.” On the web, that means building a great site with great content.
For example, if I talk about how cool a product is, and then you find out that I’m an affiliate for them, wouldn’t you as a conscientious observer become skeptical as to whether my information is biased, if perhaps I’m only saying how cool something is because I can get paid for it? Wouldn’t that make you question my integrity with other things I say as well?