The "friendliness" of the URL has nothing to do with caching by search engines, that's a problem with dynamic URLs that change for a given page over time, i.e. a blog with paginated results where old articles are pushed back onto different URLs.
The keyword ranking is A factor, but only a small one in when it comes to URLs. Other placements of keywords feature much more prominently, such as in the title of a page or the actual body text of the page.
Making your URLs friendly won't hurt, but at best the time could be better spent elsewhere if all you want is search engine results.