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.