Assigning categories to your blog is your very first chance to really help the search engines identify what your blog is all about. Use them wisely. It is at this point that you should be do some keyword research to get this right. You don't need 100 categories, generally. Most of those would be better suited as tags. Think of categories as the highest level in an outline, and do your research to make sure you use terms that people actually search for. Then, assign each post to only ONE category (the best fit). Doing so ensures that the correct category is chosen for the permalink, and you avoid duplicate content issues with posts appearing in several category listings.