Do you mean the meta keywords tag, or the use of keywords within other meta elements (e.g. a description)?
I haven't written a meta keywords tag in years (I've deleted a few though!) and I would recommend using words relevant to your page (not necessarily or even preferably keywords) within a meta description.
Figure out the top 5 keywords relating to that PAGE and throw them in the keywords tag. Though not cared for by google, there's smaller SE's and list-sites that still look at it. You can also repeat these 5 keywords TWICE in your site's alt tags (so 10 images)
I'm not sure that today keywords are working for search engines.. google nowadays is getting smarter and doesn't recognize a content by seeing its keywords even it is looking on the content, that what is content is about. If you want to recognize your site content in respect of google, you simply write smaller description by using description tag, write targeted title (page title) and use niche keywords in the content. And if you want to recognize your site globally then you may use global keyword only in the metas.