Ok, the issue with planting your seed in the final pot mainly deals with watering/feeding.
Your little, baby, seedling is not going to be able to uptake the moisture in a 3-5 gallon planter, making watering more difficult to gauge.
I do a stress-free transplant. My babies actually love when I transplant them. They immediately respond with happiness.
I start my seedlings in the red plastic party cups. I slit the cups from just below the top rim, all the way down to the very bottom. I do that all the way around the cup so I end up with around 5 or 6 slits from top to bottom. Don't cut the rim or it falls apart.
When the seedlings have started growing well and thriving, I take my 3 gallon planter, fill it full with the soil mix I use, dig out the very center the size of a red plastic party cup. Then I take a cup with a plant growing, cut the rim all the way around where the slits are, peel back the party cup on every slit. At that point I gently lift the soil (clinging to the roots) and sit it into the already dug out hole. Brush some soil around the edges, and waa laa, stress free transplant.
Make sure to water the seedlings the day before transplant. If it is too wet, the soil will fall apart, if it is too dry, the soil will fall apart.
Within hours of transplant, they always look great, leaves going at 45 degrees towards the light.