When setting up cross-domain tracking in Google Analytics, does it matter what you pick as your main site?
I recently implemented cross-domain tracking for my sites (jeffalytics.com, datadrivenu.com, my.datadrivenu.com) and it doesn't appear to have an impact that I didn't specify a primary domain to track. I left the cookies per domain and then used auto-link domains to tie them together. So far it's working as I had hoped.
Learn more about Demographics and Benchmarking in the Google Analytics Mastery Course lesson "How to Set Up Cross-Domain Tracking".