Google's John Mueller was asked in a video hangout on Google+ this morning if priority and change frequency matter much in XML Sitemaps and John from Google said nope.
The question was asked at the 29:35 mark:
Does priority and frequency matter in a sitemap? If not, how can we tell Google to crawl specific pages on daily or high priority?
Priority and change frequency doesn’t really play that much of a role with Sitemaps anymore.
This is something where we’ve tried various things but essentially, if you have a sitemap file and you are using it to tell us about the pages that were changed or updated, it is much better to just specify the time stamp directly so that we can look into our internal systems and say we haven’t crawled since this date therefore we should crawl again.
And just crawling daily doesn’t make much sense if your content doesn’t change. So that is something where we see a lot of sites they give us this information in the sitemap, they said it changes daily or weekly, and we look in our database and it hasn’t changed in a month or years…
So what I’d really recommend is using the timestamp.
This is essentially the advice Google gave when going mobile friendly, to update your Sitemap file with the time stamp of the date/time the site went mobile friendly, so Google can pick up on those changes faster.
Here is the video embed at the start time:
Forum discussion at Google+.