My tandem experience has been first with my children many years ago and recently with my grandchildren. I always run different size timing chainrings so that the children/grandchildren don't spin as fast as I do. The thinking is that it will make it easier to pedal for them. With the little ones it doesn't take much to get use to the out of phase cranks. At least in my experience it has worked extremely well for the children/grandchildren.

I can't see how there is any variation in crank speed throughout the revolution and how altering the phasing will effect it. You have basically got the momentum of a 300lb+ machine being propelled along by only a few hundred watts. Any variations are going to be very very tiny. Only on a steep hill with a big gear could I imagine it being noticeable.
We have tried in phase and 90 deg OOP. Prefer in phase, but it is just a personal thing and very easy to try out what works for you. Just give any trials enough time to get familiar with it before deciding. Probably several hundred miles.