Serenity Caldwell on iMore has a fantastic tip on how to rotate videos on an iOS device:
This one's easy to miss, but a must-have for filming fiends: iMovie for iOS is a free app for more complex video editing projects, but it also offers a quick-fix extenison in Photos for iOS. This extension lets you trim a clip, silence it, or add filters, text, or music — but it also lets you flip video with a gesture.
The rest of the article has a step-by-step guide worth checking out. This tip was news to me, but it’ll be invaluable for those times when my iPhone doesn’t orient itself before I hit record.