Absolutely premix for a new engine.
If you a just changing oil, lines or filter it’s not necessary as there’s plenty of residual oil in the crankcase.
You don’t want to run the engine with the bleed screw open because it can actually suck air into the pump.
The oil tank being higher will force oil down to the pump and with the bleed screw open let air out of the filter, lines and pump body.
Now the only thing left to purge is the little 3/32” lines from the pump to the intake.
Now is when you want to run the engine at idle. The bleed screw being closed and lever arm held wide open will let the pump build pressure pushing oil past the check valves and filling the little lines.
With the lines full the oiling system is working correctly and doesn’t require any additional oiling.