I am also getting the same error on my site http://admissions.aglasem.com
Error for Post_Tags: http://admissions.aglasem.com/post_tag-sitemap.xml
I know this is due to more memory allocation then permitted but i do not want to increase the memory size as other sitemaps are working and also other plugins are able to make the sitemap properly.
Is it possible that we can divide the tags sitemap into parts like posts.
@bpmstr - i have thousands of tags and its not fessible to go to each and select "always include". is there any way by which we can do it for all tags in one go.