It's all a matter of taste--personally, I like mine as close as possible to the strings without the strings hitting the pickup. Different pickup heights produce different timbres--experiment. One of my favorite small but significant features of Alembic electronics is that you can adjust the gain of the preamp so that the output of the bass can be adjusted to compensate for lower pickup heights. (Especially Signature, Europa, Series etc. that have independent gain adjustments for each pickup!)
If you're looking for strong attack and lots of punch, put the pickups as close as you dare to the strings. If you want a little more air in your tone lower 'em---or at least, that's my take.
