if you have submitted the sitemap once then no need to submit it again. keep adding fresh content to the blog and use the plugin for auto-updating the XML sitemap
In case of blogger, Google automatically creates sitemap for feed You can check like yourblog.blogspot.com/robots.txt You will find an sitemap that google creates automatically If you want to create a regular xml or txt sitemap than you can use xmp sitemap tool to generate it and submit it in google webmaster tool
Hi, Its is not necessary to submit sitemap if you have good internal linking and can build strong link popularity. sitemap is for those who dont know more about web crawler visibility.
I have checked for sitemap I have typed blogname.blogspot.com/robot.txt but it is giving an error in explorer. what does it mean