How important is it to have the main keyword in first sentence?

Ranking will only be affected by the popularity of the page, Google has discounted almost every meta-tag, keyword density and other "manipulative" on-page elements that can affect ranking. 2 years ago it could have made a slight difference especially if you used h1 tags but not anymore.

I don't believe in placing the exact kw in the first sentence or first paragraph either.

I recently launched a new site, x % of the posts were optimized according to Yoast SEO plugin recommendations, the other ones were written just naturally without thinking about optimizing.

The only thing that still makes a huge difference is having your kw in the title, for the rest it really doesn't matter, just make sure you mention your keyphrase at least once or twice somewhere at the page and you'll do fine.

Optimizing is nowadays much more about properly structuring your website to increase inner relevance, some things that can help with that are properly categorizing posts and internal linking. A dynamic sidebar will also help and in case you want to make it easy on yourself there is this YARPP - yet another related posts plugin, which places a link at the bottom of the post to other posts that are highly relevant based on some formula.

A good amount of content also helps very well, as it's hard to inter link without content

Wikipedia usually has their target keywords load at the top of the page (H1 + first sentence/paragraph).

Example: List of automobile manufacturers

The thing about Wikipedia on-page SEO is, they don't chase algo. updates.

Ok your keyword in first sentence this will let google know that is keyword and your article is relevant to the keyword. And if you bold the keyword it show a stronger emphasis for google that that word is your keyword.

i guess first thing to tell you is it's not important now. That ship has long sailed when you could put keywords and increase PA. Also note the wikipedia example above. Customer relationship and quality matters more than anything.

Many people say putting keyword in first and last sentence may show google that your article is relevant to the keyword as a result ranking goes up.

99% offpage . That's an anomaly. Then why did they have the Panda and Penguin update?

It is not necessary to have main keyword in the first sentence. You can use keyword in anywhere in sentence. However, when you w.rite an article, you should tell purpose of article, then we can use keyword. It is important your content is unique and quality. Besides, you should use keywords that relate to article's content.