I'm betting there is data that says take all 50/50 pitches. Unfortunately, you cannot define 50/50 pitches. A pitch is either a ball or a strike. It's just a swing thought. It's like saying a hit is as good as a walk. It would be just a saying.
But this is still a good thread.
THIS.
You can't ask hitters to think in the batter's box in terms of 50/50 pitches in the way we're talking about it here and expect that to translate into good results.
Good hitters think in terms of driving pitches they can crush (even if they're balls sometimes), and taking pitches they can't (even if they're strikes some times). They also adjust their approach to the situation at hand, so leading off with a 3-2 count should result in fewer swings for most hitters since they should usually be less willing to swing at marginal strikes in that situation (unless you're a Vlad Guerrero type). But you can't hamstring them by asking them to think in terms of 50/50 pitches and making decisions like that in the batter's box.