Variance of the Discrete Gabor Transform for practical implementation
What I look for in such parameter choice problems is empirical experiments. Whatever works best for my application is the correct value to use. 

You can develop a parameter search algorithm looking for the optimal values in some training data set. Keep in mind that what works for one application may not perform well in others. 

I'm not aware of mathematical arguments estimating  gabor's $\theta$. These estimates are typically conservative anyway.