I personally would put the pickup(s) on the instrument somewhere.
On a serious note, I would test out a few instruments in a shop to see different placements. True, they'll all give you different sounds with the different pickup shapes (p, j, hb, ex-housing etc.) but you can get a basic feel for it that way. Another thing you can do is try picking at different parts on the neck to see where you can get a desirable acoustic sound out of it. Developing an ear for bass on a record helps you figure out what sound and placement you like as well, because you can figure out what bass the artist is playing, where the pickup is and what kind of pickup it is.
Towards the bridge will give you a brighter, less bassy sound.
Toward the neck will give you a lower sound, with less distinguishable highs.
Also keep in mind, where do you feel comfortable picking? With a bass, the size of the strings makes it a little more difficult to pick consistantly, so when you switch basses, its scale, action, string spacing, guage and a bunch of other things can affect the way you play. If you're planning on playing finger style, and you plan on pivoting off of the pickup with your thumb, then take that into consideration when deciding your placement. If you really like a certain place to pivot with your thumb, but you like the sound of the pickup in another place, you can install a "thumb rest" or build one into your bass with the shape of the wood. David King does some pretty elaborate thumb rests.
Good luck!