yes, anyway is not a big deal it takes 5 min, so maybe is better to update the sitemap every time I do a change...
just let the bots crawl them, submitting a site map once is enough google bots will find them automatically. It is not so much important. Just a formality which is good.
Do you use wordpress? If yes, you can install seo plugin for it, so that it automatically submit sitemap whenever you update at frequent time.
You only need an update sitemap(the xml file),so their is no requirement to submitting it again to Google. and their is no need to resubmit sitemap on updating your site
I only resubmit if I make drastic changes to my sitemap. The reason for doing that is so that I can see how many pages are being indexed out of the ones listed in the sitemap. The google xml sitemap generator for wordpress resubmits all the time (and has had better luck getting my pages indexed ASAP). My biggest trick for asap indexing has been to link from my blog to the new static page. Google loves the links from my blog, and indexes about 70% of anything I link to.
Make it once in 2 weeks minimum. But if you add a lot of pages, or more features add ons to your site, do submit the sitemap each time you make change.