Hello,

keyword research is one of the most vital steps. You need to know your competition and how much traffic is in your niche before you start your blog. See it this way, time is money and if you waste your time with niches where is no traffic or the potential to rank high for your blog then just leave and do another research.

The best way to go is to find a product you would like to promote and go to the adwords keyword tool and google insights to see if there is traffic for that particular product. If so, then check the competition of the first page and see if you can get ranked in time with some of the keywords.

Then start your blog or leave and find another niche or product. Only start if you think that it is worthwhile to jump on this product or niche!

Hope that helps
Mirko

Surely you aren't asking whether you should do keyword research before starting a site. That's such a basic step, that I suspect you mean something else.

if you`re creating micro niche websites, you should definately check it ! lol
but if you already have some established websites you can check it also to find some low competition keywords