Hey Msinger,
Here in support, we will have you wait for all agents to upgrade in only very specific circumstances. For example, when upgrading from 6.2.1 to 6.3.1 we advised everyone to wait until all the agents we upgraded to 6.3.1 before progressing to the 6.4 upgrade. We did a pretty good overhaul on the agents back end code and communication channels from 6.2.1 to 6.3.1. And we wanted to ensure that all those agents got the needed platform files that the 6.4 + agents will need to function correctly. As for the upgrade to 6.7 there has been no reported compatibility or upgrade issues. Personally, knowing the major changes we have made to the 6.7 agents, it's possible that future upgrades will suggest making sure everything is 6.7 before upgrading but that is still currently just an assumption.
Thank you
Please let me know if I can help with anything else!
Your Friendly Neighborhood SolarWinds Support Tech!