Definitely best to masturbate before length but not girth. In length, post-masturbation stretching is much better, but for girth you will want to have the ability to get erect so be sure not to masturbate too close to girth. I personally suggest length in the AM and girth in the PM this way you can masturbate prior to stretching and then have the rest of the day to get erection quality back for your girth work.