Under normal conditions---you do not need to reprime the pump before startup. If we haven't started the engine for a week, we might spin the engine over for 15 seconds before turning on the ignition. If we haven't started the engine for 1 month, we probably would reprime.

Think about a dry sump system as two separate systems, the pressure system and the scavenge system. The scavenge system goes from the oil pan to the pump and into the top of the tank. There is no way for the oil to get from the tank back into the oil pan if it is plumbed correctly. The tank keeps the pressure section always primed---and there is no way that gravity can drain a tank UNLESS the tank is mounted well above the engine.