Especially on a PJ bass, I use the J to control brightness. You need brightness in your tone to cut through the mix. Neck only tone may sound great when you practice at home but in the band and especially in live situation it gets lost in the mix quickly especially indoor, as most venues don’t have acoustic treatment and the bass frequencies get trapped in the corners of the wall and ceilings, mids and treble adjustment help reduces that effect a bit.
My G&L SB2 have no tone knob altogether same goes to my Joe Dart II both only have Vol/ Vol. so the jazz volume knob acts as a tone knob.
On my Jazz bass the pickups are very hot and I frequently turn both down to 90%. Otherwise, set them however you like the sound. If that means setting the P pickup to 80%, definitely do that.
The main rule is that you do what it takes to get a sound you like.
Muscle memory and experience taught me to “max” my volume knob at 80% sometime you just need that little extra boost and the mix engineer can’t give it to you fast enough. I need both boost and cut.