I've always found using both holds the best results. You get the full round bottom end with a sharp attack, I find that if I switch between each solely I get either bottom or just sharp boxy attack
I find it mostly depends on the bass in question, but I never use the bridge PU solo'ed. It's either just the neck pick-up or neck&bridge if I need the extra-clear highs. I almost always prefer passive pickups on basses, the active ones tend to sound very flat and without character for me. Oh, and I always prefer single coils to humbuckers, they tend to sound better, to me at least. But given that all my heroes use a Precision bass, it's kind of self-explanatory why I love the neck/singlecoil (split-P) combo.
Most often both but I have had cases where leaning more to one of the pickups sounds better. Let him play while you experiment with different positions on the knobs and decide what sounds best.
I often find that favouring the bridge pickup cuts through better in the mix but often times can't admit to that because most of the time I prefer the tone of both pickups blended equally. both pickups tend to blend in better and give fullness. I've never used the neck pickup in a modern metal setting but for other styles a p-bass'ish tone can be great.
If it's humbuckers, always bridge. Especially with fast material there's just so much more focus and definition to the low end.
I had a long conversation with a real bass expert who runs a high-end bass shop here in Leamington and he said that he found that the better the playing, the bass, the amp and the cab all were, the less he ever uses a blend of pickups. After testing out a variety of equipment in his shop I'm inclined to agree.
I still use both (barts) but it's worth noting that bass pickup location varies more than it does on guitar. It's not not even the same on all jazz basses.