The lower the crossover point, the less music there is going to your subs (and the more music there is going to your mains). The higher the crossover point, the more music there is going to your subs (and the less music there is going to your mains).
Keep in mind that setting the crossover point too high (anything much higher than about 80 Hz) can result in the localization of bass frequencies, which tends to diminish any imaging/staging you may have had.
There really is no "optimal' setting - its all about preference. Try out a few different settings with your favorite tunes and stick with the setting that sounds the best to you.
Bang on.