There are a lot of different factors that go into SEO, and domain names are one of them. For instance, if parked domains are actively used and contain the same content as your primary domain, search engines may penalize your websites for duplicate content. However, if you set up redirects from all of your parked domains to your primary, you can leave your parked domains empty – and should someone try accessing one of them, they will be redirected to your primary website.
As for addon domains, when used correctly, they can have a positive effect on your SEO. Take your blog, for example. If you create an addon domain for your blog, you can publish fresh content without cluttering your website. This will give you an opportunity to rank for different keywords, which can help drive more traffic to your primary website.
The same applies to your subdomains. Since they are usually considered entirely different entities, they become opportunities for optimization and ranking. The key is to make sure you aren’t posting repetitive or duplicate content. If you find that either your addon domains or subdomains are duplicate in nature, then your best option for SEO is to set up redirects.