For your website stick you your niche. If you have a motivational thought or something you think others might like, then post it to WA. Or if you have a tutorial, then post it to WA. Always good practice posting to WA.

What you do on your website is your business. Having said that, if people are expecting to read about cars and you write about trains, you will lose them. Jim

Essentially the aim of your website is to attract an audience with a specific interest in something, that way you can promote targeted products to them - so posting "any article" wouldn't really help you build any sort of specific audience... Instead it would likely just confuse your visitors. If there are other topics you'd like to write about you could always create another website & write about them there. You get up to 50 websites with your membership here at WA :)

Stick to the training and Yes, always post on your website relating to your niche as this way you will gain trust from your visitors and authority
Hope this helps
Vicki