On your website, not necessarily. It's a good idea to continue to write content on your website the reinforces the main keyword. Google wants to know what your website is about and a good way to do that is to write content that is related to your main keyword.

I typically write a main content piece for my homepage (main keyword) then an additional XX amount of content that supports that keyword and lets Google know what the website is about.

For instance:

Homepage: Apples
Additional Content: Washington Apples, Red Delicious Apples, Green Apples, Etc...

As opposed to...

Homepage: Apple
Additional Content: Iphone 5, Macbook Pro, ICloud, etc...

Other alternative is:

put your main keyword in keyword tool and get a list of related keywords.. write article based on those keywords, you can make changes in those keywords to lengthen as well..

If you are doing article submission, then its a good idea to post different article of one keywords, as google love fresh content nowadays

I agree with all the comments above. For on-page, you do not need to include your main kw/s in every content. You may however, do this by adding synonymous kw/s so it won't look over optimized. As for off page, you may include your target kw/s in every content as you will be submitting these to different sites anyway.