Of course not. These two files are not required all the time.
In Google Webmaster Tools, Google says that a robots.txt is not necessary if you want that all webpages being crawled. If you want to put only these following lines in a robots.txt:
Just don't create a robots.txt.
Moreover, a sitemap.xml helps indexing of your website but it's not obliged to use it. Googlebot is smart enough to follow links on the web and when it finds a link to one of your webpage, it follows it and crawls your webpage to index it. For example, if you have many backlinks, Googlebot will crawl often your webpages and you don't need to create a sitemap.xml.