We know the priority value is not required, as per the specification. But it doesn't stop those submitting sitemaps from "worrying" if they should specify something of value in there.
I wouldn't worry too much about the "priority" values. If you're able to generate them based on useful information from your website, then it's a good idea to do that. If you can't automatically generate them and have too many pages to set the values manually, then it's fine to submit the Sitemap file without "priority" information. The "priority" value gives us a bit more information about your site, but changing values here generally won't lead to visible changes in crawling, indexing, or ranking overall.
It is always hard to know which pages to give higher priority to. Some like to give the home page and then category pages higher priority, while other webmasters take the reverse approach.
What do you do?
Forum discussion at Google Webmaster Help.