Is robots.txt important?After I have uploaded my sitemap file, should I still make a robots.txt file?
This two are different concepts and the answer is yes you require a robots.txt file even though you have sitemap file which will help robots to search properly. robots.txt is useful if you want to restrict robot entry to some of your sites pages
Sitemap and robots.txt are very different, sitemap is made to let google discovering new pages and robots.txt is made to restrict access to some areas of your site for search engines
robot.txt is always require even if you have the sitemap, because sitemap is for indexing your webpages properly on search engine and through robot.txt you can define that which part of your website should be crawl or not by google bots.
Sitemaps help googlebot find deep pages it wouldn't have found while the robots.txt file says, hey googlebot,dont crawl these files!
To further complicate the topic, there is also a robot meta tags. In my opinion, it wouldn't hurt if you are still going to use a robots.txt. If Google or other search engine said it is bad then don't use it.
By having the robots.txt file you can still prevent bots from using up resources on your server. Also google doesnt like crawling pages that it doesn't have to - google webmaster guidelines still state using robots.txt to restrict crawler access is advisable.