Generally speaking if you do not change many things in your blog, then you should not need to worry about your blog malfunctioning. On the other hand if you never experiment with changing your theme or other files then you might never attain the knowledge you would be able to acquire. If you never add plugins to your site, then there is a lower risk something bad might happen, but your blog will not have as many useful features as your competitors blogs. Basically the more active you are with your site, the bigger chance something will malfunction. It is a good idea to check if the key aspects of your blog are working, because you would not want to have any problems because you neglected to check some elements of your blog. I recommend checking at least the most important thing like: contact page, about section, adsense ads, blogroll, affiliate links and feeds. You can never be too careful.