The purpose of meta tags is to describe your content. Search Engines of course pick up your title and description and use them as you main explanation of the page.
When you Should
If you like to write in a way that does not include an overview of your blog post or page for that matter in the first paragraph, I would suggest adding one. This will allow you to put a full overview in the 165 character description tag and tell searchers what the entire post is about.
Additionally, if you feel like your first paragraph does explain your post but is too long to give it justice in the meta description then by all means edit it.
When you Should Not
If your intro paragraph adequately describes the post content I would suggest not changing it. Search Engines hat manufactured content for ranking purposes only, so having some natural ones in there in my opinion will help quite a bit.
Even if you tell search bots what to index as your meta description there is still a very real chance your first paragraph will get picked up instead. We still have many service pages on our site that contain the first paragraph as our page description despite our repeated attempts to update it.
Play it safe and try to let your blog posts get naturally indexed with the content on the page. If you really feel like your first paragraph isn’t good enough then manually add the content through a plugin like Yoast. Just make sure you aren’t trying to keyword stuff your way to the top or it could be the quickest way you lose rankings on your site with search engines.