The choice of θ largely determines the amount of shear reinforcement. If you want to minimize the amount of shear reinforcement, you should choose the lowest allowed θ-value.
But you also have to take the increase in longitudinal reinforcement into account. Low θ-values require more longitudinal reinforcement and cause anchoring problems at the beam ends. Often resulting in additional bars to meet the anchoring conditions.
Thus a low θ-value doesn’t result in an economical optimal concrete strut.
The results of practical and theoretical reaseach on this matter have been published in a paper from R. Manuoi and J. Rondal, “Optimal shear design of beams with CEB-FIB Model Code‘. From this paper the choice θ = 31° appears to be the most optimal solution.