... with all algorithm updates and changes almost nothing is 100% certain... I've seen a lot of competitors without neither sitemaps nor robots.txt and they rank on first page...
Length of time in a top position of the rankings is a bit like insulation, the same way recognised brands get treated differently than startups. If you or I were to build backlinks like some big brands have done, we'd be hammered so hard by Google, we'd have to change our name to Nail or Spike.
Google have kindly published a bit of information about what they want/expect from a website:
http://static.googleusercontent.com.../search-engine-optimization-starter-guide.pdf
While in principle what fisicx is saying is completely true, I personally don't advocate that site owners go with the 'least required' approach. If the time required to do something is trivial, and if there's a small chance that it could positively impact your rankings, visitor counts or engagement, or improve sales - then what else have you got better to do than invest that small amount of time in doing things the best you possibly can do them?
If you ever get a manual review (perhaps due to a competitor making bogus claims about you), do you want a human examining your site and noting that although minimum requirements were met, minimal recommendations were not?
By the way, page 11 of that guide points out what Google recommends about sitemaps, and page 21 discusses robots.txt.
Focus on getting your site error free, falling in line with the recommendations from Google, adding remarkable content on a regular basis, syndicating socially and on occasion get some backlinks - the ones that take more than 3 seconds to build
Cheers!