In addition to allowing polls to reveal themselves at the start time of the session, which we already do, we should have a second setting, somewhat like the visible/not visible "eye" icon for discussions, that allows speakers and admins to make a poll visible or not visible, even after the session is live.
This would allow hosts to roll out poll questions one at a time, and either hide the ones that are inactive, showing just one question at a time, or allow the poll questions to build up as the session goes on. Either way, the attendees would be unable to see polling questions before they are supposed to interact with them. I've seen this method requested a number of times for educational-type sessions.
Recommendation: allow this setting to be accessible in both the Host Tools Dashboard, for easy setup, and the host-facing "Manage" tab for sessions.