What ever it takes to maintain about 50 lbs margin on the gauge wheels and keep ground contact at 99-100%.  On my Precision equipped planter that generally means I'm using the Standard setting at 100 lbs to up to 130 lbs.  I don't actually know what this number means because it is often applying 150-200 lbs/row unit.  In hard ground and/or heavy trash situations requires a higher number.  As for wear on bearings......I think I've replaced 18 bearings this spring but none of them have been on openers or depth gauge wheels.  Closing wheels and row cleaners have been my issue since I went to high speed.  FWIW.....maintenance costs are higher on high speed machines.