Will one be penalized for having every file in XML Sitemap listed with the same priority? Definitely, one will not be penalized for this. If one gives same importance to all files in XML Sitemap, then Google will try to find out which ones are really important from their perspective. So, one need not to worry about having to list a priority for every single one. Google is not going to have any sort of scoring or whatever. It is completely fine and optional as well. So, if one has some details for why they wish to put it in a priority, then it is really great. But, to be honest, it is not a must-one and it will really not get one into trouble, if you don't have it.