I don't pack. I fill my 5g gallon fabric pots all the way to the top and SLIGHTLY tamp down with pre-watered soil (using the squeeze test) , and when I say slightly, I mean slightly tamp down. Then about flip time or a couple weeks after the soil should be settled a bit. Maybe 2". Then I'll top with fresh soil to the very top and tamp that down a bit.That way when I water next couple times those extra fresh nutrients, minerals, and microbes will filter into the soil a bit making up for what was used during veg. Then proceed with Terp Tea Bloom as a supplement every other watering up till 3 weeks before finish.
That's how I do it. Everyone's different....