We get these questions a lot!
It is not about keywords and the manipulation of keywords. It has never been the case since the beginning of Google and never will be. So stop. Please. This common SEO advice is utterly ridiculous and made out of complete ignorance of how search engines actually work.
Here is what you need to know.
Search engines are semantic engines and have been for a very long time. It is not about keyword matching and has not been since Google came on the scene in 1997/8.
What you want to do is stop thinking in terms of keywords and where and how to place them. You are doing yourself more harm thinking this way. Why? Because fully formed naturally written sentences outperform anything.
Prior to Google search engines performed some predictive analysis along with metrics associated with term matches and term placement in relation to each other and the beginning of the HTML document. In some form, semantics begins here, however, semantics is not term matching at all, but linguistics analysis and algorithmic scoring in matrices. It is this scoring that allows search engines to match search intent. The search engine is not matching keywords, it is matching topic relevancy based upon language and the written word. While this is not technically difficult to understand, it is far more detailed in scope than a simple keyword match and predictive analysis. Semantics and how it is applied gets complex really really fast.
Semantics is not new. It existed as a fully formed analysis engine back in the early to mid-1970s. There have been some new semantic analysis algorithms since then, however, most are predicated on preexisting analysis. Very little is actually new except how it is applied and how inventive the web allows semantic analysis to be. It is really remarkable the depth of how semantics is applied these days. We are very far away from the days of keyword matches.
To answer your question.
Write fully formed sentences everywhere possible. Not staccato headlines. Not keyword loaded stuff. It is not necessary. Search engines understand what you are writing better than you can ever imagine. Your h1 tag should apply to the topic of the whole page while any other header tag should apply to the topic of the content immediately following it. What is important are subject, predicate, and object just like you studied in your English classes. If you write fully formed sentences, this is a natural thing you already do. You do not even have to think about it.
Remember to create content for humans and not machines. Part of semantic analysis is determining when what is written is for machines. Some level of analysis can determine if a page is designed for search engines and your content may gain some negative scores as a result. Semantics prefers sentences. One reason for this is that smaller terms can really help understand the topic. For example, I recently described how so-called stop words (and there is no such thing anymore) help search engines understand content. Using the word a means singular. While this seems trivial, it becomes a powerful semantic clue at times where a term is used that does not have a plural version.
Creating content for readers helps with so-called long-tail search. Long-tail is a bit of a misnomer. It never really existed except in the keyword chase world. Those who wrote naturally, never had to think in these terms. It was simply a natural by-product. However, those who chased keywords, constantly struggle against the current. It is important to use the right terms of course. For that, you should be aware of the terms used for any topic. In this, search engines can determine expertise and will use expertise levels to make search query matches.
So write naturally written sentences as much as you can and be sure to know your topic and you will be fine. You will not have to think too much. Let the search engines do the thinking for you. After all, are you smarter than almost two decades of engineering by countless linguistics experts at Google? Short answer? No. None of us are. So do not try and out think them. It does not pay.