IMHO you should fly at Vy (Vx for obstacle clearance if necessary). that provides the highest altitude should you suffer an engine failure.
Piper recommend this for the Seminole too I think. However, I teach to climb at an attitude corresponding to the more conventional Vyse+10kts or so, because that is the same attitude as a Vyse climb on one engine. That is, there's no significant attitude change required if an engine fails.
If I have an obstacle to clear, then I will maintain Vy or Vx until a safe height, as per manufacturer's recommendation. But I specifically remind myself that I'll need to lower the nose dramatically in the event of an EF. This will also necessitate turning early or landing ahead because I won't clear the obstacle. Not a "normal" take-off (for me at least!)
I too am interested in whether it damages the engine to apply mixture ICO -> fully rich with the throttle open. It's a pity no-one seems to know.
Thanks to all the very experienced pilots who have made posts here - CC & Centaurus among others. I (we?) have appreciated your taking time to pass on your knowledge.