If you're concerned about getting an erection when you see her, then masturbating beforehand will probably reduce the chances of that slightly. If you're hoping to get lucky and want to last longer, it might help there also.
The main danger I foresee is that the masturbation session beforehand involves you imagining all sorts of inappropriate acts with her, which could then colour the way you interact with her, and your expectations of the date (ie if you've just been picturing yourself shagging her in a nurse's outfit against a wall, it might be difficult to get back into the mindset of gentlemanly behaviour).
Other than that, knock yourself out.