people hang the plants upside down for one main reason, its convienent... they can dry, out of the way, doing their thing, and you dont have to worry about turning them or moving them and they arent laying down losing trichs... they dry evenly this way due to the fact air can move freely around them... if you trim the buds while they are wet and still on the plant you can hang and dry a whole plant with trimmed buds that can dry without much worry of mold, again due to air movement... it has nothing to do with THC being in the stem.. thats not true... you can cure them in glass or in plastic... either works... mason jars are typically used because you can get a lot for relatively little money, they are air tight when closed which will help the curing process, and you can place the colas in the right side up which helps eliminate trich loss... just make sure to burp them a few times a day... there are many ways to dry and cure.. its like taking a trip.. you have to decide which way getting from point A (wet buds) to point B (smoking those buds) works best for you and gives you the best smoke... experiment with a couple different ways and see which you like best... i personally cut off individual limbs and trim them wet, hang them for about 5 days to dry, and cure them for about 5-7 in mason jars... i have used plastic to cure it... i just personally like the glass jars BECAUSE i lost fewer trichs.. it seemed that laying them down in the plastic container i used made them move around more and i lost more trichs overall.... just mho tho