I don't keep my horses manes in braids because it made mine fall out!! I braided it lossely and took it out a couple of days later and some of his mane came out with the braids! Thank goodness this mane is thick and you could not tell.I brush it every day( But I put leave in conditioner in so the brush dose not ripe all of his mane and tail out!) I know every one says don't brush there manes and tail if you want it to be long and pretty but I tryed this and when I went to get all the tangles out I riped put a ton of hair that sould not have came out so I brush it every day WITH come sort of leave in conditioner. I have seen some barns leve in braid for months and take it out and the horses mane was fine but that is not the case for my horse.
With a really long mane (I've worked with Friesians in the past), you definitely want to braid the mane into a few thick strands and bag it. When it comes to a more "regular" long mane, as long as you're maintaining the mane (and it sounds like you are, MTG is my lifesaver) and keep an eye out to make sure you're not losing any hair, it's really up to the owner's discretion.