We generally start our OTA rollout with a small percent (~10%, randomly chosen) of our users to ensure that the OTA system is working correctly. The reason we do this is so that in case we find any serious issues, we can contain the problem quickly and find a solution before it impacts a wider audience. This also allows us the address the affected users quickly without too much of a queue. We usually open up OTAs to the general public 24-48 hours after the initial release if all goes well.