I have an script which I found online and then modified a bit which pings Google, Yahoo, Bing and ASK. I use it every time I have new content or content updated (new reviews on my site).
My sitemaps are accurate. I mean: they offer the creation date of the content or the last modified/updated date. This gives bots useful information about the my pages and helps them deciding if they have to crawl again or not.
I have had no problems and have doing this way for more than a year.
From what I know, it is OK to ping Google to update your sitemap:
- if you have something new - not more than every 1 hour