When finding the potential energy of a dipole in a uniform electric field, I was told by my lecturer that the convention is that the potential energy is 0 when the dipole moment and electric field vectors are perpendicular. I don't understand why this is so, when it would make sense if the potential was zero when the dipole moment and electric field are parallel by agreeing with the minimum total potential energy principle (that is, a body will displace to minimise its total potential energy).
The only reason I could possibly think for this convention is that the expression is simplified from $U=pE(1-cos\theta)$ to $U=-pEcos\theta$. If anyone can give me a better reason for this convention, it would really clear things up for me. Thanks.